145. Binary Tree Postorder Traversal

/**
 * 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> res=new ArrayList<>();
        if(root==null){
            return res;
        }
        helper(res,root);
        return res;
    }
    public void helper(List<Integer> res,TreeNode root){
        if(root==null){
            return;
        }
        helper(res,root.left);
        helper(res,root.right);
        res.add(root.val);
    }
}