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  by :

　　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 }