solutions/0437-path-sum-iii.js

/**
 * 437. Path Sum III
 * https://leetcode.com/problems/path-sum-iii/
 * Difficulty: Medium
 *
 * Given the root of a binary tree and an integer targetSum, return the number of paths where the
 * sum of the values along the path equals targetSum.
 *
 * The path does not need to start or end at the root or a leaf, but it must go downwards
 * (i.e., traveling only from parent nodes to child nodes).
 */

/**
 * 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
 * @param {number} sum
 * @return {number}
 */
var pathSum = function(root, sum) {
  if (!root) return 0;
  return (subPathSum(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum));
};

function subPathSum(node, sum) {
  if (!node) return 0;
  const self = node.val === sum ? 1 : 0;
  return self + subPathSum(node.left, sum - node.val) + subPathSum(node.right, sum - node.val);
}