[LeetCode 解题报告]226. Invert Binary Tree

Invert a binary tree.

Example:

Input:

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

Output:

     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 f*** off.

Method 1. 

/**
 * 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 == NULL)
            return NULL;
        
        TreeNode* curLeft = root->left;
        root->left = invertTree(root->right);
        root->right = invertTree(curLeft);
        return root;
    }
};

 Method 2.

/**
 * 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 == NULL)
            return NULL;
        
        queue<TreeNode*> que({root});
        while (!que.empty()) {
            TreeNode* cur = que.front();
            que.pop();
            
            TreeNode* tmp = cur->left;
            cur->left = cur->right;
            cur->right = tmp;
            
            if (cur->left)
                que.push(cur->left);
            if (cur->right)
                que.push(cur->right);
        }
        return root;
    }