/*
* August 23, 2015
*
* Binary tree post order iterative solution
*
*/
/*
* source code from blog:
*
* http://articles.leetcode.com/2010/10/binary-tree-post-order-traversal.html
*
* Analysis from the above blog:
*
* F
* / \
* B G
* / \ \
* A D I
* / \ /
* C E H
*
* Post-order traversal sequence: A, C, E, D, B, H, I, G, F
*
* We use a prev variable to keep track of the previously-traversed node.
*
* Let’s assume curr is the current node that’s on top of the stack.
* When prev is curr's parent, we are traversing down the tree.
* In this case,
* we try to traverse to curr's left child if available
* (ie, push left child to the stack).
* If it is not available, we look at curr's right child.
* If both left and right child do not exist (ie, curr is a leaf node), we
* print curr's value and pop it off the stack.
If prev is curr's left child, we are traversing up the tree from
* the left. We look at curr's right child. If it is available, then traverse
* down the right child (ie, push right child to the stack), otherwise print
* curr's value and pop it off the stack.
If prev is curr's right child, we are traversing up the tree from
* the right.
* In this case, we print curr's value and pop it off the stack.
*
* julia's comment:
* 1. Count 3 cases,
* case 1: prev is curr's parent
* case 2: prev is curr's left child
* case 3: prev is curr's right child
* 2. convert C++ code to C#
*
*/
public static ArrayList postOrderTraversalIterative(Node root)
{
ArrayList res = new ArrayList();
if (root == null)
return res;
Stack s = new Stack();
s.Push(root);
Node prev = null;
while (s.Count >0)
{
Node curr = (Node)s.Peek();
if (prev == null || prev.left == curr || prev.right == curr) // case 1 (going downward): prev is a parent node, like root node
{
if (curr.left != null)
s.Push(curr.left);
else if (curr.right != null)
s.Push(curr.right);
}
else if (curr.left == prev) // case 2 (left child->parent->right child, stage 2)
{
if (curr.right != null) // if there is a right child, get child into stack
s.Push(curr.right);
}
else // prev is curr.right, and then, output the node, pop it from stack; since children are visited.
{
res.Add(Convert.ToChar(curr.val));
s.Pop();
}
prev = curr; // important, set curr to prev - do not forget!
}
return res;
}
static void Main(string[] args)
{
Node n1 = new Node('F');
n1.left = new Node('B');
n1.right = new Node('G');
n1.left.left = new Node('A');
n1.left.right = new Node('D');
n1.left.right.left = new Node('C');
n1.left.right.right = new Node('E');
n1.right.right = new Node('I');
n1.right.right.left = new Node('H');
ArrayList test = postOrderTraversalIterative(n1);
}