1.翻转二叉树
图片说明
思路：递归

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(root==nullptr) return root;
        TreeNode* left=invertTree(root->left);
        TreeNode* right=invertTree(root->right);//递归左右子树
        root->left=right;
        root->right=left;//交换
        return root;
    }
};