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.
class Solution {
public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
Map<Integer, Integer> map = new HashMap();
for (int a : A) {
for (int b : B) {
map.put(a + b, map.getOrDefault(a + b, 0) + 1);
}
}
int count = 0;
for (int c : C) {
for (int d : D) {
count += map.getOrDefault(- c - d, 0);
}
}
return count;
}
}