Binary Tree Postorder Traversal - Solution I.java

import java.util.ArrayList;
import lintcode.util.TreeNode;
import java.util.Stack;
/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> rst = new ArrayList<Integer>();
        if (root == null) {
            return rst;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        TreeNode cur = root;
        TreeNode prev = null;
        while (!stack.isEmpty()) {
            cur = stack.peek();
            if (prev == null || prev.left == cur || prev.right == cur) {
                if (cur.left != null) {
                    stack.push(cur.left);
                } else if (cur.right != null) {
                    stack.push(cur.right);
                }
            } else if (cur.left == prev) {
                if (cur.right != null) {
                    stack.push(cur.right);
                }
            } else {
                rst.add(cur.val);
                stack.pop();
            }
            prev = cur;
        }
        return rst;
    }
}