LeetCode 226. Invert Binary Tree

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode invertTree(TreeNode root) {        if (root == null) return null;        TreeNode tmp = root.left;        root.left = root.right;        root.right = tmp;        invertTree(root.left);        invertTree(root.right);        return root;    }}