翻转二叉树（简单）
递归。
/*** 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;}TreeNode left=invertTree(root.left);TreeNode right=invertTree(root.right);root.left=right;root.right=left;return root;}
}