226. Invert Binary Tree (easy)

Q:
Invert a binary tree.

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

to

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

A:
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)
            return root;
        auto l = root->left;
        auto r = root->right;
        invertTree(l);
        invertTree(r);
        root->left = r;
        root->right = l;
        return root;
    }
};