LeetCode OJ：Invert Binary Tree（反转二叉树）

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.

反转二叉树，递归的执行反转就行了：

 /**
  * 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 NULL;
         swap(root->left, root->right);
         root->left = invertTree(root->left);
         root->right = invertTree(root->right);
         return root;
     }
 };

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）：

 public class Solution {
     public TreeNode invertTree(TreeNode root) {
         if(root == null)
             return root;
         TreeNode leftNew = invertTree(root.right);
         TreeNode rightNew = invertTree(root.left);
         root.left = leftNew;
         root.right = rightNew;
         return root;
     }
 }