# Letcode 454 - 4sum II

`Note:`

- It looks terrifying but actually it’s way more easier than
`4Sum I`

. - Use a hash table
`map`

to store all different sums of`nums1`

,`nums2`

, and their frequencies. (Store frequencies because each`-sum`

from`nums3`

and`nums4`

can combine with the pair whose key is`sum`

in map.) - Then use another for loop to check all possible
`sum`

and check if it’s been stored in`map`

as key`-sum`

. The frequency is how many combination we can get for each`sum`

.

`Question:`

Given four integer arrays `nums1`

, `nums2`

, `nums3`

, and `nums4`

all of length n, return the number of tuples (i, j, k, l) such that:

- 0 <= i, j, k, l < n
- nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0

`Example:`

1 | Input: nums1 = [1,2], nums2 = [-2,-1], nums3 = [-1,2], nums4 = [0,2] |

`Code:`

1 | /** |