【LeetCode】145. 二叉树的后序遍历

给定一个二叉树，返回它的 后序 遍历。

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 
输出: [3,2,1]

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