二叉树的后续遍历

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList
     */
    public ArrayList<Integer> postorderTraversal (TreeNode root) {
        // write code here
        ArrayList<Integer> res = new ArrayList<>();

        if(root == null) return res;


        if(root.left != null){
            res.addAll(postorderTraversal(root.left));
        };

        if(root.right != null){
            res.addAll(postorderTraversal(root.right));
        };

        res.add(root.val);

        return res;
    }
}