1
var pathSum = function (root, sum, presums = { 0: 1 }, prev = 0) {
2
  if (!root) return 0;
3
  let curr = prev + root.val;
4
  presums[curr] = (presums[curr] || 0) + 1;
5
  let res = (presums[curr - sum] || 0) - !sum;
6
  res +=
7
    pathSum(root.left, sum, presums, curr) +
8
    pathSum(root.right, sum, presums, curr);
9
  presums[curr]--;
10
  return res;
11
};