LeetCode 454 4 Sum II - Medium

454. 4 Sum II — Medium

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Problem

454. 4Sum II

Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.

To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.

Example:

Input: A = [ 1, 2] B = [-2,-1] C = [-1, 2] D = [ 0, 2]

Output: 2

Explanation: The two tuples are:

1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0

Solution

class Solution:
    def fourSumCount(self, A, B, C, D):
        counter = {}
        for a in A:
            for b in B:
                counter[a + b] = counter.get(a + b, 0) + 1
        answer = 0
        for c in C:
            for d in D:
                answer += counter.get(-c - d, 0)
        return answer