Question Definition
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
Java Solution
public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
Map<Integer, Integer> map = new HashMap<>();
for(int i = 0; i < A.length; i++){
for(int j = 0; j < B.length; j++){
map.putIfAbsent(A[i] + B[j], 0);
map.put(A[i] + B[j], map.get(A[i] + B[j]) + 1);
}
}
int result = 0;
for(int i = 0; i < C.length; i++){
for(int j = 0; j < D.length; j++){
if(map.containsKey(-C[i] - D[j]))
result += map.get(-C[i] - D[j]);
}
}
return result;
}
Comments