BinaryTreePostorderTraversal.java

import java.util.ArrayList;
import java.util.Stack;

/**
 * Given a binary tree, return the postorder traversal of its nodes' values.
 *
 * For example:
 * Given binary tree {1,#,2,3},
 *   1
 *    \
 *     2
 *    /
 *   3
 * return [3,2,1].
 *
 * Note: Recursive solution is trivial, could you do it iteratively?
 * 
 * Solution:
 * 
 * 1. Root - Right - Left
 * 2. Then reverse the sequence
 * 3. Left - Right - Root
 *
 */

public class BinaryTreePostorderTraversal {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
    	ArrayList<Integer> ret = new ArrayList<Integer>();
		if (root == null)
			return ret;
		Stack<TreeNode> st = new Stack<TreeNode>();
		TreeNode p = root.right;
		ret.add(root.val);
		st.add(root);
		while (!st.isEmpty()) {
			while (p != null) {
				ret.add(p.val);
				st.add(p);
				p = p.right;
			}
			TreeNode node = st.pop();
			p = node.left;
			if (p != null) {
				ret.add(p.val);
				st.add(p);
				p = p.right;
			}
		}
		int i = 0;
		int j = ret.size() - 1;
		while(i < j) {
			int tmp = ret.get(i);
			ret.set(i, ret.get(j));
			ret.set(j, tmp);
			i++;
			j--;
		}
		return ret;
    }
}