const pathSum = function (root, sum) {
  if (root === null) return 0;
  return pathSumFrom(root, sum) + pathSum(root.left, sum) + pathSum(root.right, sum); // 先序遍历树


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