leetcode-go/medium/454.4sum-ii.go

func fourSumCount(A []int, B []int, C []int, D []int) (cnt int) {
	n := len(A)
	m := make(map[int]int)
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			m[C[i]+D[j]]++
		}
	}
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			if val, ok := m[-A[i]-B[j]]; ok {
				cnt += val
			}
		}
	}
	return
}