454.4Sum II

1- Map O(n^2)

class Solution {
    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        int n = A.length;
        int cnt = 0;
        Map<Integer, Integer> sums = new HashMap<>();
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                int sum = A[i] + B[j];
                sums.put(sum, sums.getOrDefault(sum, 0) + 1);
            }
        }

        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++){
                int sum = C[i] + D[j];
                if(sums.containsKey(-sum))
                    cnt += sums.get(-sum);
            }
        }
        return cnt;
    }
}