LeetCode OJ：Invert Binary Tree（反转二叉树）

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

     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.

反转二叉树，递归的执行反转就行了：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* invertTree(TreeNode* root) {
13         if(!root) return NULL;
14         swap(root->left, root->right);
15         root->left = invertTree(root->left);
16         root->right = invertTree(root->right);
17         return root;
18     }
19 };

java版本的如下所示（由于不方便使用swap，所以这里采取的另一种的方式）：

 1 public class Solution {
 2     public TreeNode invertTree(TreeNode root) {
 3         if(root == null)
 4             return root;
 5         TreeNode leftNew = invertTree(root.right);
 6         TreeNode rightNew = invertTree(root.left); 
 7         root.left = leftNew;
 8         root.right = rightNew;
 9         return root;
10     }
11 }