LeetCode: 3sum With Multiplicity Solution

Approach

Create a hash table counting occurences of two sum

Implementation

1/**
2 * @param {number[]} arr
3 * @param {number} target
4 * @return {number}
5 */
6var threeSumMulti = function (arr, target) {
7  const N = arr.length
8  const MOD = 1e9 + 7
9  const map = new Map()
10
11  let res = 0
12
13  for (let i = 0; i < N; i++) {
14    res = (res + (map.get(target - arr[i]) || 0)) % MOD
15
16    for (let j = 0; j < i; j++) {
17      const twoSum = arr[i] + arr[j]
18      map.set(twoSum, (map.get(twoSum) || 0) + 1)
19    }
20  }
21
22  return res
23}