一：题目

二:上码(前序解法)

/*** 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 TreeNode invertTree(TreeNode root) {if (root == null) return null;swap(root);//交换结点的时候 用的是root 这样才能改变root的结构invertTree(root.left);invertTree(root.right);return root;}public void swap(TreeNode node) {TreeNode temp = null;temp = node.left;node.left = node.right;node.right = temp;}
}