Easy 226题 Invert Binary Tree

问题：

Invert a binary tree.

     4   /   \  2     7 / \   / \1   3 6   9

to

     4   /   \  7     2 / \   / \9   6 3   1

/** * 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)      {          TreeNode* tmp=root->left;          root->left=root->right;          root->right=tmp;                    invertTree(root->left);          invertTree(root->right);      }      return root;    }};