Problem

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.

Solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
  if (root) {
    swap(root);
    invertTree(root.left);
    invertTree(root.right);
  }
  return root;
};

var swap = function (node) {
  var left = node.left;
  node.left = node.right;
  node.right = left;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).