226. Invert Binary Tree
给一棵二元树，
要把树上的每个节点的左右子节点都交换。
Example 1:
Input: root = [4, 2, 7, 1, 3, 6, 9]
Output: [4, 7, 2, 9, 6, 3, 1]
Explanation:
https://assets.leetcode.com/uploads/2021/03/14/invert1-tree.jpg
Example 2:
Input: root = [2, 1, 3]
Output: [2, 3, 1]
Explanation:
https://assets.leetcode.com/uploads/2021/03/14/invert2-tree.jpg
Example 3:
Input: root = []
Output: []
Explanation:
树上没有任何节点，直接返回空的树
解题思路:
递回处理左右子节点，
然后把当前节点的左右子节点交换。
C++ code:
class Solution {
public:
TreeNode* invertTree(TreeNode* root) {
if(!root) return NULL;
TreeNode *temp = root->right;
root->right = invertTree(root->left);
root->left = invertTree(temp);
return root;
}
};