Leetcode no. 145

145. 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?

/**
 * 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) {
        LinkedList<Integer> list= new LinkedList<>();
            if (root==null) return list;
            Stack<TreeNode> stack= new Stack<>();
            stack.push(root);
            while (!stack.isEmpty()){
                TreeNode t= stack.pop();
                list.addFirst(t.val);
                if (t.left!=null) stack.push(t.left);
                if (t.right!=null) stack.push(t.right);
            }
            return list;
    }
}