Leetcode[226]-Invert Binary Tree

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.

递归法求解：

/**
 * 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;

        invertNode(root);
        return root;
    }

    void invertNode(TreeNode *root){
        if(root == NULL) return;
        if(!root->left && !root->right) return;

        TreeNode *tempNode=NULL;

        if(root->right) tempNode = root->right;
        if(root->left){
            root->right = root->left;
            root->left = tempNode;
        }else{
            root->left = tempNode;
            root->right = NULL;
        }
        invertNode(root->left);
        invertNode(root->right);

    }
};