invert-binary-tree.js

// 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.


/**
 * 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) return null;
    var temp = root.left;
    root.left = root.right;
    root.right = temp;
    if (root.left) invertTree(root.left);
    if (root.right) invertTree(root.right);
    return root;
};