454. 4Sum II

题目分析

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.

时间复杂度为 O(n ^ 2)，空间复杂度为 O(n)，解该题用到了 HashMap。

代码

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