LC 1995. 统计特殊四元组（哈希表）

func countQuadruplets(nums []int) int {
    n := len(nums)
    mp := make([]int, 301)
    res := 0
    for c := n - 2; c >= 2; c-- {
        mp[nums[c+1]]++
        for a := 0; a < c - 1; a++ {
            for b := a + 1; b < c; b++ {
                res += mp[nums[a] + nums[b] + nums[c]]
            }
        }
    } 
    return res
}