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;
    }
}