LeetCode 136: Invert Binary Tree

Invert a binary tree.

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

to

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

题目中的这句话亮了：

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.

这题很简单，下面是我的答案：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* invertTree(struct TreeNode* root) {
    if(NULL == root) return root;
    invertTree(root->left);
    invertTree(root->right);
    struct TreeNode * temp = root->left;
    root->left = root->right;
    root->right = temp;
    return root;
}

后来又简化了一下：

struct TreeNode* invertTree(struct TreeNode* root) {
    if(NULL == root) return root;
    struct TreeNode * preright = root->right;
    root->right = invertTree(root->left);
    root->left = invertTree(preright);
    return root;
}