leetcode 之 invert binary tree

Invert a binary tree.

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

to

4
/   \
7     2
/ \   / \

9 6 3 1

解法：
就是简单的递归
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if(root == NULL)
            return NULL;
        TreeNode *l = invertTree(root->left);
        TreeNode *r = invertTree(root->right);
        TreeNode *tmp = l;
        root->left = r;
        root->right = tmp;
        return root;
    }
};