0222-count-complete-tree-nodes.js

/**
 * 222. Count Complete Tree Nodes
 * https://leetcode.com/problems/count-complete-tree-nodes/
 * Difficulty: Easy
 *
 * Given the root of a complete binary tree, return the number of the nodes in the tree.
 *
 * According to Wikipedia, every level, except possibly the last, is completely filled
 * in a complete binary tree, and all nodes in the last level are as far left as possible.
 * It can have between 1 and 2h nodes inclusive at the last level h.
 *
 * Design an algorithm that runs in less than O(n) time complexity.
 */

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var countNodes = function(root) {
  return !root ? 0 : 1 + countNodes(root.left) + countNodes(root.right);
};