226 Invert Binary Tree(easy)

1.直接用递归把左右子树翻转即可（仅针对二叉树的情况）。

2.时间复杂度为O(n)，即遍历所有节点一次

Invert a binary tree.

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

to

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

Trivia:
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.

[c language=”++”]
/**
* 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:
void invert(TreeNode* root)
{
if (root != NULL)
{
swap(root->left, root->right);
invert(root->left);
invert(root->right);
}

}
TreeNode* invertTree(TreeNode* root) {
invert(root);
return root;
}

};
[/c]