454_4Sum_II.java

class Solution {
    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        int answer = 0;
        Map<Integer, Integer> AB = getSumMap(A, B);
        Map<Integer, Integer> CD = getSumMap(C, D);

        for (int ABSum : AB.keySet()) {
            if (CD.containsKey(-1 * ABSum)) {
                answer += (AB.get(ABSum) * CD.get(-1 * ABSum));
            }
        }
        return answer;
    }

    Map<Integer, Integer> getSumMap(int[] array1, int[] array2) {
        Map<Integer, Integer> sumMap = new HashMap<>();
        for (int i = 0; i < array1.length; ++i) {
            for (int j = 0; j < array2.length; ++j) {
                int sum = array1[i] + array2[j];
                if (!sumMap.containsKey(sum)) {
                    sumMap.put(sum, 1);
                    continue;
                }
                sumMap.replace(sum, sumMap.get(sum) + 1);
            }
        }
        return sumMap;
    }
}