Leetcode: 4Sum II

4Sum II



Similar Problems:


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

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.


// Blog link: https://code.dennyzhang.com/4sum-ii
// Basic Ideas: hashmap
// Complexity: Time O(n*n), Space O(n*n)
func fourSumCount(A []int, B []int, C []int, D []int) int {
    m := map[int]int{}
    for _, v1 := range A {
        for _, v2:= range B {
            m[v1+v2]++
        }
    }

    res := 0
    for _, v1 := range C {
        for _, v2:= range D {
            res += m[-v1-v2]
        }
    }
    return res
}
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