0129-sum-root-to-leaf-numbers.js

/**
 * 129. Sum Root to Leaf Numbers
 * https://leetcode.com/problems/sum-root-to-leaf-numbers/
 * Difficulty: Medium
 *
 * You are given the root of a binary tree containing digits from 0 to 9 only.
 *
 * Each root-to-leaf path in the tree represents a number.
 *
 * - For example, the root-to-leaf path 1 -> 2 -> 3 represents the number 123.
 *
 * Return the total sum of all root-to-leaf numbers. Test cases are generated
 * so that the answer will fit in a 32-bit integer.
 *
 * A leaf node is a node with no children.
 */

/**
 * 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 sumNumbers = function(root) {
  function traverse(node, value) {
    if (!node) {
      return null;
    }
    value += node.val;
    if (!node.left && !node.right) {
      return Number(value);
    }
    return traverse(node.left, value) + traverse(node.right, value);
  }

  return traverse(root, '');
};