Binary Tree Postorder Traversal

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
       public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> ret = new ArrayList<Integer>();
        if(root == null) return ret;


        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);

        while(!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            ret.add(cur.val);

            if(cur.left != null) {
                stack.push(cur.left);
            }

            if(cur.right != null) {
                stack.push(cur.right);
            }
        }

        Collections.reverse(ret);
        return ret;
    }
}