226. Invert Binary Tree

/*Invert a binary tree.     4   /   \  2     7 / \   / \1   3 6   9to     4   /   \  7     2 / \   / \9   6 3   1Trivia:This problem was inspired by this original tweet by Max Howell:Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.*//** * 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) return root;        TreeNode* tmp=root->left;        root->left=root->right;        root->right=tmp;        invertTree(root->left);        invertTree(root->right);        return root;    }};