//DFS: 后序 Post-order Traversal, Stack
public List<int> PostOrderTraversal_Iterative1(TreeNode root)
{
//Postorder traversal is more complicated to implement comparing to Preorder and Inorder.
//At what point shall we visit the root node:
//Condition 1: The root node has no child at all.
//Condition 2: The root's child has already been visited.
List<int> result = new List<int>();
Stack<TreeNode> stack = new Stack<TreeNode>();
if (root != null)
stack.Push(root);
TreeNode prev = null;
while (stack.Count != 0)
{
root = stack.Peek();
bool noChild = (root.left == null && root.right == null);
bool doneChild = false;
if (prev != null && (prev == root.left || prev == root.right))
doneChild = true;
//直到没有child了或者child已经访问完了才把栈顶元素出栈
if (noChild || doneChild)
{
root = stack.Pop();
result.Add(root.val);
prev = root;
}
else
{
if (root.right != null)
stack.Push(root.right);
if (root.left != null)
stack.Push(root.left);
}
}
return result;
}
public int solution(string S)
{
int minInteger = 0;
int maxInteger = (1 << 20) - 1;
Stack<int> stack = new Stack<int>();
foreach (string op in S.Split(' '))
{
if (op == "DUP")
{
if (!stack.Any())
{
return -1;
}
stack.Push(stack.Peek());
}
else if (op == "POP")
{
if (!stack.Any())
{
return -1;
}
stack.Pop();
}
else if (op == "+")
{
if (stack.Count() < 2)
{
return -1;
}
var top = stack.Pop();
var nextTop = stack.Pop();
var sum = top + nextTop;
if (sum > maxInteger)
{
// Overflow
return -1;
}
stack.Push(sum);
}
else if (op == "-")
{
if (stack.Count() < 2)
{
return -1;
}
var top = stack.Pop();
var nextTop = stack.Pop();
var diff = top - nextTop;
if (diff < minInteger)
{
// Overflow
return -1;
}
stack.Push(diff);
}
else
{
int number;
if (!int.TryParse(op, out number))
{
return -1;
}
if (number < minInteger || number > maxInteger)
{
return -1;
}
stack.Push(number);
}
}
if (!stack.Any())
{
return -1;
}
return stack.Pop();
}
