private void dfs(Graph G, int v)
{
marked[v] = true;
foreach (int w in G.Adj(v))
{
// short circuit if odd-length cycle found
if (cycle != null)
{
return;
}
// found uncolored vertex, so recur
if (!marked[w])
{
edgeTo[w] = v;
color[w] = !color[v];
dfs(G, w);
}
// if v-w create an odd-length cycle, find it
else if (color[w] == color[v])
{
isBipartite = false;
cycle = new LinkedStack <int>();
cycle.Push(w); // don't need this unless you want to include start vertex twice
for (int x = v; x != w; x = edgeTo[x])
{
cycle.Push(x);
}
cycle.Push(w);
}
}
}
// does this graph have two parallel edges?
// side effect: initialize cycle to be two parallel edges
private bool hasParallelEdges(Graph G)
{
marked = new bool[G.V];
for (int v = 0; v < G.V; v++)
{
// check for parallel edges incident to v
foreach (int w in G.Adj(v))
{
if (marked[w])
{
cycle = new LinkedStack <int>();
cycle.Push(v);
cycle.Push(w);
cycle.Push(v);
return(true);
}
marked[w] = true;
}
// reset so marked[v] = false for all v
foreach (int w in G.Adj(v))
{
marked[w] = false;
}
}
return(false);
}
// check that algorithm computes either the topological order or finds a directed cycle
private void dfs(Digraph G, int v)
{
onStack[v] = true;
marked[v] = true;
foreach (int w in G.Adj(v))
{
// short circuit if directed cycle found
if (cycle != null)
{
return;
}
else if (!marked[w])
{
// found new vertex, so recur
edgeTo[w] = v;
dfs(G, w);
}
else if (onStack[w])
{
// trace back directed cycle
cycle = new LinkedStack <int>();
for (int x = v; x != w; x = edgeTo[x])
{
cycle.Push(x);
}
cycle.Push(w);
cycle.Push(v);
Debug.Assert(check());
}
}
onStack[v] = false;
}
private void dfs(Graph G, int u, int v)
{
marked[v] = true;
foreach (int w in G.Adj(v))
{
// short circuit if cycle already found
if (cycle != null)
{
return;
}
if (!marked[w])
{
edgeTo[w] = v;
dfs(G, v, w);
}
// check for cycle (but disregard reverse of edge leading to v)
else if (w != u)
{
cycle = new LinkedStack <int>();
for (int x = v; x != w; x = edgeTo[x])
{
cycle.Push(x);
}
cycle.Push(w);
cycle.Push(v);
}
}
}
private LinkedStack <int> cycle = null; // Eulerian cycle; null if no such cylce
/// <summary>
/// Computes an Eulerian cycle in the specified digraph, if one exists.
/// </summary>
/// <param name="G">the digraph</param>
///
public DirectedEulerianCycle(Digraph G)
{
// must have at least one edge
if (G.E == 0)
{
return;
}
// necessary condition: indegree(v) = outdegree(v) for each vertex v
// (without this check, DFS might return a path instead of a cycle)
for (int v = 0; v < G.V; v++)
{
if (G.Outdegree(v) != G.Indegree(v))
{
return;
}
}
// create local view of adjacency lists, to iterate one vertex at a time
IEnumerator <int>[] adj = new IEnumerator <int> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = G.Adj(v).GetEnumerator();
}
// initialize stack with any non-isolated vertex
int s = nonIsolatedVertex(G);
LinkedStack <int> stack = new LinkedStack <int>();
stack.Push(s);
// greedily add to putative cycle, depth-first search style
cycle = new LinkedStack <int>();
while (!stack.IsEmpty)
{
int v = stack.Pop();
while (adj[v].MoveNext())
{
stack.Push(v);
v = adj[v].Current;
}
// add vertex with no more leaving edges to cycle
cycle.Push(v);
}
// check if all edges have been used
// (in case there are two or more vertex-disjoint Eulerian cycles)
if (cycle.Count != G.E + 1)
{
cycle = null;
}
Debug.Assert(certifySolution(G));
}
private void bfs(Graph G, int s)
{
LinkedQueue <int> q = new LinkedQueue <int>();
color[s] = WHITE;
marked[s] = true;
q.Enqueue(s);
while (!q.IsEmpty)
{
int v = q.Dequeue();
foreach (int w in G.Adj(v))
{
if (!marked[w])
{
marked[w] = true;
edgeTo[w] = v;
color[w] = !color[v];
q.Enqueue(w);
}
else if (color[w] == color[v])
{
isBipartite = false;
// to form odd cycle, consider s-v path and s-w path
// and let x be closest node to v and w common to two paths
// then (w-x path) + (x-v path) + (edge v-w) is an odd-length cycle
// Note: distTo[v] == distTo[w];
cycle = new LinkedQueue <int>();
LinkedStack <int> stack = new LinkedStack <int>();
int x = v, y = w;
while (x != y)
{
stack.Push(x);
cycle.Enqueue(y);
x = edgeTo[x];
y = edgeTo[y];
}
stack.Push(x);
while (!stack.IsEmpty)
{
cycle.Enqueue(stack.Pop());
}
cycle.Enqueue(w);
return;
}
}
}
}
/// <summary>
/// Returns a path between the source vertex <c>s</c> and vertex <c>v</c>, or
/// <c>null</c> if no such path.</summary>
/// <param name="v">the vertex</param>
/// <returns>the sequence of vertices on a path between the source vertex
/// <c>s</c> and vertex <c>v</c>, as an IEnumerable</returns>
///
public IEnumerable <int> PathTo(int v)
{
if (!HasPathTo(v))
{
return(null);
}
LinkedStack <int> path = new LinkedStack <int>();
for (int x = v; x != s; x = edgeTo[x])
{
path.Push(x);
}
path.Push(s);
return(path);
}
private void dfs(Digraph G, int v)
{
marked[v] = true;
low[v] = pre++;
int min = low[v];
stack.Push(v);
foreach (int w in G.Adj(v))
{
if (!marked[w])
{
dfs(G, w);
}
if (low[w] < min)
{
min = low[w];
}
}
if (min < low[v])
{
low[v] = min;
return;
}
int u;
do
{
u = stack.Pop();
id[u] = count;
low[u] = G.V;
} while (u != v);
count++;
}
public static void MainTest(string[] args)
{
int M = int.Parse(args[0]);
MinPQ <Transaction> pq = new MinPQ <Transaction>(M + 1);
TextInput StdIn = new TextInput();
while (!StdIn.IsEmpty)
{
// Create an entry from the next line and put on the PQ.
string line = StdIn.ReadLine();
Transaction transaction = new Transaction(line);
pq.Insert(transaction);
// remove minimum if M+1 entries on the PQ
if (pq.Count > M)
{
pq.DelMin();
}
} // top M entries are on the PQ
// print entries on PQ in reverse order
LinkedStack <Transaction> stack = new LinkedStack <Transaction>();
foreach (Transaction transaction in pq)
{
stack.Push(transaction);
}
foreach (Transaction transaction in stack)
{
Console.WriteLine(transaction);
}
}
private void dfs(Digraph G, int v)
{
marked[v] = true;
preorder[v] = pre++;
stack1.Push(v);
stack2.Push(v);
foreach (int w in G.Adj(v))
{
if (!marked[w])
{
dfs(G, w);
}
else if (id[w] == -1)
{
while (preorder[stack2.Peek()] > preorder[w])
{
stack2.Pop();
}
}
}
// found strong component containing v
if (stack2.Peek() == v)
{
stack2.Pop();
int w;
do
{
w = stack1.Pop();
id[w] = count;
} while (w != v);
count++;
}
}
// additional test
static void TopInts()
{
int[] allInts = { 12, 11, 8, 7, 9, 5, 4, 3, 2, 29, 23, 1, 24, 30, 9, 4, 88, 5, 100, 29, 23, 5, 99, 87, 22, 111 };
MinPQ <int> pq0 = new MinPQ <int>(allInts);
int M = allInts.Length / 3;
MinPQ <int> pq = new MinPQ <int>(M + 1);
Console.WriteLine("Top {0} is ", M);
foreach (var n in allInts)
{
pq.Insert(n);
Console.WriteLine("Min is {0}", pq.Min);
// remove minimum if M+1 entries on the PQ
if (pq.Count > M)
{
pq.DelMin();
}
}
// print entries on PQ in reverse order
LinkedStack <int> stack = new LinkedStack <int>();
foreach (int n in pq)
{
stack.Push(n);
}
foreach (int n in stack)
{
Console.WriteLine(n);
}
Console.WriteLine("These are the top elements");
}
// does this graph have a self loop?
// side effect: initialize cycle to be self loop
private bool hasSelfLoop(Graph G)
{
for (int v = 0; v < G.V; v++)
{
foreach (int w in G.Adj(v))
{
if (v == w)
{
cycle = new LinkedStack <int>();
cycle.Push(v);
cycle.Push(v);
return(true);
}
}
}
return(false);
}
private readonly int s; // source vertex
/// <summary>
/// Computes the vertices connected to the source vertex <c>s</c> in the graph <c>G</c>.</summary>
/// <param name="G">the graph</param>
/// <param name="s">the source vertex</param>
///
public NonrecursiveDFS(Graph G, int s)
{
this.s = s;
marked = new bool[G.V];
edgeTo = new int[G.V];
// to be able to iterate over each adjacency list, keeping track of which
// vertex in each adjacency list needs to be explored next
IEnumerator <int>[] adj = new IEnumerator <int> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = G.Adj(v).GetEnumerator();
}
// depth-first search using an explicit stack
LinkedStack <int> stack = new LinkedStack <int>();
marked[s] = true;
stack.Push(s);
//Console.Write("Visiting ({0})\n", s);
while (!stack.IsEmpty)
{
int v = stack.Peek();
if (adj[v].MoveNext())
{
int w = adj[v].Current;
//Console.Write("Approaching {0}\n", w);
if (!marked[w])
{
// discovered vertex w for the first time
marked[w] = true;
edgeTo[w] = v;
stack.Push(w);
//Console.Write("Visiting ({0})\n", w);
}
//else
// Console.Write("Visited {0}\n", w);
}
else
{
//Console.Write("Returning from {0}\n", stack.Peek());
stack.Pop();
}
}
}
/// <summary>
/// Returns the extreme points on the convex hull in counterclockwise order.</summary>
/// <returns>the extreme points on the convex hull in counterclockwise order</returns>
///
public IEnumerable <Point2D> Hull()
{
LinkedStack <Point2D> s = new LinkedStack <Point2D>();
foreach (Point2D p in hull)
{
s.Push(p);
}
return(s);
}
/// <summary>
/// Returns the vertices in reverse postorder.</summary>
/// <returns>the vertices in reverse postorder, as an iterable of vertices</returns>
///
public IEnumerable <int> ReversePost()
{
LinkedStack <int> reverse = new LinkedStack <int>();
foreach (int v in postorder)
{
reverse.Push(v);
}
return(reverse);
}
private bool[] marked; // marked[v] = is there an s->v path?
/// <summary>
/// Computes the vertices reachable from the source vertex <c>s</c> in the
/// digraph <c>G</c>.</summary>
/// <param name="G">the digraph</param>
/// <param name="s">the source vertex</param>
///
public NonrecursiveDirectedDFS(Digraph G, int s)
{
marked = new bool[G.V];
// to be able to iterate over each adjacency list, keeping track of which
// vertex in each adjacency list needs to be explored next
IEnumerator <int>[] adj = new IEnumerator <int> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = G.Adj(v).GetEnumerator();
}
// depth-first search using an explicit stack
LinkedStack <int> stack = new LinkedStack <int>();
marked[s] = true;
stack.Push(s);
while (!stack.IsEmpty)
{
int v = stack.Peek();
if (adj[v].MoveNext())
{
int w = adj[v].Current;
// Console.Write("check {0}\n", w);
if (!marked[w])
{
// discovered vertex w for the first time
marked[w] = true;
// edgeTo[w] = v;
stack.Push(w);
// Console.Write("dfs({0})\n", w);
}
}
else
{
// Console.Write("{0} done\n", v);
stack.Pop();
}
}
}
private int M; // number of characters in regular expression
/// <summary>Initializes the NFA from the specified regular expression.</summary>
/// <param name="regexp">the regular expression</param>
/// <exception cref="ArgumentException">if the regular expression is invalid</exception>
///
public NFA(string regexp)
{
this.regexp = regexp;
M = regexp.Length;
LinkedStack <int> ops = new LinkedStack <int>();
G = new Digraph(M + 1);
for (int i = 0; i < M; i++)
{
int lp = i;
if (regexp[i] == '(' || regexp[i] == '|')
{
ops.Push(i);
}
else if (regexp[i] == ')')
{
int or = ops.Pop();
// 2-way or operator
if (regexp[or] == '|')
{
lp = ops.Pop();
G.AddEdge(lp, or + 1);
G.AddEdge(or, i);
}
else if (regexp[or] == '(')
{
lp = or;
}
else
{
Debug.Assert(false);
}
}
// closure operator (uses 1-character lookahead)
if (i < M - 1 && regexp[i + 1] == '*')
{
G.AddEdge(lp, i + 1);
G.AddEdge(i + 1, lp);
}
if (regexp[i] == '(' || regexp[i] == '*' || regexp[i] == ')')
{
G.AddEdge(i, i + 1);
}
}
if (ops.Count != 0)
{
throw new ArgumentException("Invalid regular expression");
}
}
/// <summary>
/// Returns a longest path from the source vertex <c>s</c> to vertex <c>v</c>.</summary>
/// <param name="v">the destination vertex</param>
/// <returns>a longest path from the source vertex <c>s</c> to vertex <c>v</c></returns>
/// as an iterable of edges, and <c>null</c> if no such path
///
public IEnumerable <DirectedEdge> PathTo(int v)
{
if (!HasPathTo(v))
{
return(null);
}
LinkedStack <DirectedEdge> path = new LinkedStack <DirectedEdge>();
for (DirectedEdge e = edgeTo[v]; e != null; e = edgeTo[e.From])
{
path.Push(e);
}
return(path);
}
// check that algorithm computes either the topological order or finds a directed cycle
private void dfs(EdgeWeightedDigraph G, int v)
{
onStack[v] = true;
marked[v] = true;
foreach (DirectedEdge e in G.Adj(v))
{
int w = e.To;
// short circuit if directed cycle found
if (cycle != null)
{
return;
}
else if (!marked[w])
{
//found new vertex, so recur
edgeTo[w] = e;
dfs(G, w);
}
else if (onStack[w])
{
// trace back directed cycle
cycle = new LinkedStack <DirectedEdge>();
DirectedEdge eTemp = e;
while (eTemp.From != w)
{
cycle.Push(eTemp);
eTemp = edgeTo[eTemp.From];
}
cycle.Push(eTemp);
return;
}
}
onStack[v] = false;
}
/// <summary>
/// Returns a shortest path between the source vertex <c>s</c> and vertex <c>v</c>.</summary>
/// <param name="v">the destination vertex</param>
/// <returns>a shortest path between the source vertex <c>s</c> and vertex <c>v</c>;
/// <c>null</c> if no such path</returns>
///
public IEnumerable <Edge> PathTo(int v)
{
if (!HasPathTo(v))
{
return(null);
}
LinkedStack <Edge> path = new LinkedStack <Edge>();
int x = v;
for (Edge e = edgeTo[v]; e != null; e = edgeTo[x])
{
path.Push(e);
x = e.Other(x);
}
return(path);
}
/// <summary>
/// Initializes a new graph that is a deep copy of <c>G</c>.</summary>
/// <param name="G">the graph to copy</param>
///
public Graph(Graph G) : this(G.V)
{
numEdges = G.E;
for (int v = 0; v < G.V; v++)
{
// reverse so that adjacency list is in same order as original
LinkedStack <int> reverse = new LinkedStack <int>();
foreach (int w in G.adj[v])
{
reverse.Push(w);
}
foreach (int w in reverse)
{
adj[v].Add(w);
}
}
}
/// <summary>
/// Returns a shortest path from vertex <c>s</c> to vertex <c>t</c>.</summary>
/// <param name="s">the source vertex</param>
/// <param name="t">the destination vertex</param>
/// <returns>a shortest path from vertex <c>s</c> to vertex <c>t</c>
/// as an iterable of edges, and <c>null</c> if no such path</returns>
/// <exception cref="InvalidOperationException">if there is a negative cost cycle</exception>
///
public IEnumerable <DirectedEdge> Path(int s, int t)
{
if (HasNegativeCycle)
{
throw new InvalidOperationException("Negative cost cycle exists");
}
if (!HasPath(s, t))
{
return(null);
}
LinkedStack <DirectedEdge> path = new LinkedStack <DirectedEdge>();
for (DirectedEdge e = edgeTo[s, t]; e != null; e = edgeTo[s, e.From])
{
path.Push(e);
}
return(path);
}
public static void MainTest(string[] args)
{
LinkedStack <string> s = new LinkedStack <string>();
TextInput StdIn = new TextInput();
while (!StdIn.IsEmpty)
{
string item = StdIn.ReadString();
if (!item.Equals("-"))
{
s.Push(item);
}
else if (!s.IsEmpty)
{
Console.Write(s.Pop() + " ");
}
}
Console.WriteLine("(" + s.Count + " left on stack)");
}
/// <summary>
/// Computes the convex hull of the specified array of points.
/// </summary>
/// <param name="pts">the array of points</param>
/// <exception cref="NullReferenceException">if <c>points</c> is <c>null</c> or if any
/// entry in <c>points[]</c> is <c>null</c></exception>
///
public GrahamScan(Point2D[] pts)
{
// defensive copy
int N = pts.Length;
Point2D[] points = new Point2D[N];
for (int i = 0; i < N; i++)
{
points[i] = pts[i];
}
// preprocess so that points[0] has lowest y-coordinate; break ties by x-coordinate
// points[0] is an extreme point of the convex hull
// (alternatively, could do easily in linear time)
Array.Sort(points);
// sort by polar angle with respect to base point points[0],
// breaking ties by distance to points[0]
Array.Sort(points, 1, N - 1, points[0].GetPolarOrder());
hull.Push(points[0]); // p[0] is first extreme point
// find index k1 of first point not equal to points[0]
int k1;
for (k1 = 1; k1 < N; k1++)
{
if (!points[0].Equals(points[k1]))
{
break;
}
}
if (k1 == N)
{
return; // all points equal
}
// find index k2 of first point not collinear with points[0] and points[k1]
int k2;
for (k2 = k1 + 1; k2 < N; k2++)
{
if (Point2D.Ccw(points[0], points[k1], points[k2]) != 0)
{
break;
}
}
hull.Push(points[k2 - 1]); // points[k2-1] is second extreme point
// Graham scan; note that points[N-1] is extreme point different from points[0]
for (int i = k2; i < N; i++)
{
Point2D top = hull.Pop();
while (Point2D.Ccw(hull.Peek(), top, points[i]) <= 0)
{
top = hull.Pop();
}
hull.Push(top);
hull.Push(points[i]);
}
Debug.Assert(isConvex());
}
private LinkedStack <int> path = null; // Eulerian path; null if no suh path
/// <summary>
/// Computes an Eulerian path in the specified digraph, if one exists.</summary>
/// <param name="G">the digraph</param>
///
public DirectedEulerianPath(Digraph G)
{
// find vertex from which to start potential Eulerian path:
// a vertex v with outdegree(v) > indegree(v) if it exits;
// otherwise a vertex with outdegree(v) > 0
int deficit = 0;
int s = nonIsolatedVertex(G);
for (int v = 0; v < G.V; v++)
{
if (G.Outdegree(v) > G.Indegree(v))
{
deficit += (G.Outdegree(v) - G.Indegree(v));
s = v;
}
}
// digraph can't have an Eulerian path
// (this condition is needed)
if (deficit > 1)
{
return;
}
// special case for digraph with zero edges (has a degenerate Eulerian path)
if (s == -1)
{
s = 0;
}
// create local view of adjacency lists, to iterate one vertex at a time
IEnumerator <int>[] adj = new IEnumerator <int> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = G.Adj(v).GetEnumerator();
}
// greedily add to cycle, depth-first search style
LinkedStack <int> stack = new LinkedStack <int>();
stack.Push(s);
path = new LinkedStack <int>();
while (!stack.IsEmpty)
{
int v = stack.Pop();
while (adj[v].MoveNext())
{
stack.Push(v);
v = adj[v].Current;
}
// push vertex with no more available edges to path
path.Push(v);
}
// check if all edges have been used
if (path.Count != G.E + 1)
{
path = null;
}
Debug.Assert(check(G));
}
private LinkedStack <int> cycle; // the directed cycle; null if digraph is acyclic
/// <summary>
/// Determines whether the digraph <c>G</c> has a directed cycle and, if so,
/// finds such a cycle.</summary>
/// <param name="G">the digraph</param>
///
public DirectedCycleX(Digraph G)
{
// indegrees of remaining vertices
int[] indegree = new int[G.V];
for (int v = 0; v < G.V; v++)
{
indegree[v] = G.Indegree(v);
}
// initialize queue to contain all vertices with indegree = 0
LinkedQueue <int> queue = new LinkedQueue <int>();
for (int v = 0; v < G.V; v++)
{
if (indegree[v] == 0)
{
queue.Enqueue(v);
}
}
for (int j = 0; !queue.IsEmpty; j++)
{
int v = queue.Dequeue();
foreach (int w in G.Adj(v))
{
indegree[w]--;
if (indegree[w] == 0)
{
queue.Enqueue(w);
}
}
}
// there is a directed cycle in subgraph of vertices with indegree >= 1.
int[] edgeTo = new int[G.V];
int root = -1; // any vertex with indegree >= -1
for (int v = 0; v < G.V; v++)
{
if (indegree[v] == 0)
{
continue;
}
else
{
root = v;
}
foreach (int w in G.Adj(v))
{
if (indegree[w] > 0)
{
edgeTo[w] = v;
}
}
}
if (root != -1)
{
// find any vertex on cycle
bool[] visited = new bool[G.V];
while (!visited[root])
{
visited[root] = true;
root = edgeTo[root];
}
// extract cycle
cycle = new LinkedStack <int>();
int v = root;
do
{
cycle.Push(v);
v = edgeTo[v];
} while (v != root);
cycle.Push(root);
}
Debug.Assert(check());
}
private int[] distTo; // distTo[v] = number of edges on shortest path to v
/// <summary>
/// Determines a maximum matching (and a minimum vertex cover)
/// in a bipartite graph.</summary>
/// <param name="G">the bipartite graph</param>
/// <exception cref="ArgumentException">if <c>G</c> is not bipartite</exception>
///
public HopcroftKarp(Graph G)
{
bipartition = new BipartiteX(G);
if (!bipartition.IsBipartite)
{
throw new ArgumentException("graph is not bipartite");
}
// initialize empty matching
this.V = G.V;
mate = new int[V];
for (int v = 0; v < V; v++)
{
mate[v] = UNMATCHED;
}
// the call to hasAugmentingPath() provides enough info to reconstruct level graph
while (hasAugmentingPath(G))
{
// to be able to iterate over each adjacency list, keeping track of which
// vertex in each adjacency list needs to be explored next
IEnumerator <int>[] adj = new IEnumerator <int> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = G.Adj(v).GetEnumerator();
}
// for each unmatched vertex s on one side of bipartition
for (int s = 0; s < V; s++)
{
if (IsMatched(s) || !bipartition.Color(s))
{
continue; // or use distTo[s] == 0
}
// find augmenting path from s using nonrecursive DFS
LinkedStack <int> path = new LinkedStack <int>();
path.Push(s);
while (!path.IsEmpty)
{
int v = path.Peek();
// retreat, no more edges in level graph leaving v
if (!adj[v].MoveNext())
{
path.Pop();
}
// advance
else
{
// process edge v-w only if it is an edge in level graph
int w = adj[v].Current;
if (!isLevelGraphEdge(v, w))
{
continue;
}
// add w to augmenting path
path.Push(w);
// augmenting path found: update the matching
if (!IsMatched(w))
{
// Console.WriteLine("augmenting path: " + toString(path));
while (!path.IsEmpty)
{
int x = path.Pop();
int y = path.Pop();
mate[x] = y;
mate[y] = x;
}
cardinality++;
}
}
}
}
}
// also find a min vertex cover
inMinVertexCover = new bool[V];
for (int v = 0; v < V; v++)
{
if (bipartition.Color(v) && !marked[v])
{
inMinVertexCover[v] = true;
}
if (!bipartition.Color(v) && marked[v])
{
inMinVertexCover[v] = true;
}
}
Debug.Assert(certifySolution(G));
}
/// <summary>
/// Computes an Eulerian path in the specified graph, if one exists.</summary>
/// <param name="G">the graph</param>
///
public EulerianPath(Graph G)
{
// find vertex from which to start potential Eulerian path:
// a vertex v with odd degree(v) if it exits;
// otherwise a vertex with degree(v) > 0
int oddDegreeVertices = 0;
int s = nonIsolatedVertex(G);
for (int v = 0; v < G.V; v++)
{
if (G.Degree(v) % 2 != 0)
{
oddDegreeVertices++;
s = v;
}
}
// graph can't have an Eulerian path
// (this condition is needed for correctness)
if (oddDegreeVertices > 2)
{
return;
}
// special case for graph with zero edges (has a degenerate Eulerian path)
if (s == -1)
{
s = 0;
}
// create local view of adjacency lists, to iterate one vertex at a time
// the helper Edge data type is used to avoid exploring both copies of an edge v-w
LinkedQueue <Edge>[] adj = new LinkedQueue <Edge> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = new LinkedQueue <Edge>();
}
for (int v = 0; v < G.V; v++)
{
int selfLoops = 0;
foreach (int w in G.Adj(v))
{
// careful with self loops
if (v == w)
{
if (selfLoops % 2 == 0)
{
Edge e = new Edge(v, w);
adj[v].Enqueue(e);
adj[w].Enqueue(e);
}
selfLoops++;
}
else if (v < w)
{
Edge e = new Edge(v, w);
adj[v].Enqueue(e);
adj[w].Enqueue(e);
}
}
}
// initialize stack with any non-isolated vertex
LinkedStack <int> stack = new LinkedStack <int>();
stack.Push(s);
// greedily search through edges in iterative DFS style
path = new LinkedStack <int>();
while (!stack.IsEmpty)
{
int v = stack.Pop();
while (!adj[v].IsEmpty)
{
Edge edge = adj[v].Dequeue();
if (edge.IsUsed)
{
continue;
}
edge.IsUsed = true;
stack.Push(v);
v = edge.Other(v);
}
// push vertex with no more leaving edges to path
path.Push(v);
}
// check if all edges are used
if (path.Count != G.E + 1)
{
path = null;
}
Debug.Assert(certifySolution(G));
}
/// <summary>
/// Computes an Eulerian cycle in the specified graph, if one exists.</summary>
/// <param name="G">the graph</param>
///
public EulerianCycle(Graph G)
{
// must have at least one edge
if (G.E == 0)
{
return;
}
// necessary condition: all vertices have even degree
// (this test is needed or it might find an Eulerian path instead of cycle)
for (int v = 0; v < G.V; v++)
{
if (G.Degree(v) % 2 != 0)
{
return;
}
}
// create local view of adjacency lists, to iterate one vertex at a time
// the helper Edge data type is used to avoid exploring both copies of an edge v-w
LinkedQueue <Edge>[] adj = new LinkedQueue <Edge> [G.V];
for (int v = 0; v < G.V; v++)
{
adj[v] = new LinkedQueue <Edge>();
}
for (int v = 0; v < G.V; v++)
{
int selfLoops = 0;
foreach (int w in G.Adj(v))
{
// careful with self loops
if (v == w)
{
if (selfLoops % 2 == 0)
{
Edge e = new Edge(v, w);
adj[v].Enqueue(e);
adj[w].Enqueue(e);
}
selfLoops++;
}
else if (v < w)
{
Edge e = new Edge(v, w);
adj[v].Enqueue(e);
adj[w].Enqueue(e);
}
}
}
// initialize stack with any non-isolated vertex
int s = nonIsolatedVertex(G);
LinkedStack <int> stack = new LinkedStack <int>();
stack.Push(s);
// greedily search through edges in iterative DFS style
cycle = new LinkedStack <int>();
while (!stack.IsEmpty)
{
int v = stack.Pop();
while (!adj[v].IsEmpty)
{
Edge edge = adj[v].Dequeue();
if (edge.IsUsed)
{
continue;
}
edge.IsUsed = true;
stack.Push(v);
v = edge.Other(v);
}
// push vertex with no more leaving edges to cycle
cycle.Push(v);
}
// check if all edges are used
if (cycle.Count != G.E + 1)
{
cycle = null;
}
Debug.Assert(certifySolution(G));
}