• Question
• Solution

Question

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.

Solution

Result: Accepted Time: 0 ms

Here should be some explanations.

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

Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$