四数相加II

import "sort"
func fourSumCount(A []int, B []int, C []int, D []int) int {
length := len(A)
if length < 1{
return 0
}

groupA, groupB, groupC, groupD := NewGroup(A), NewGroup(B), NewGroup(C), NewGroup(D)
timesOfSum := make(map[int]int)    

for _, nodeA := range groupA{
    for _, nodeB := range groupB{
        sumOfAB := nodeA.Val + nodeB.Val
        addTimes := nodeA.Times * nodeB.Times
        times, existed := timesOfSum[sumOfAB]
        if existed{
            times += addTimes
        }else{
            times = addTimes
        }
        timesOfSum[sumOfAB] = times
    }
}

total := 0
for _, nodeC := range groupC{
    for _, nodeD := range groupD{
        sumOfCD := nodeC.Val + nodeD.Val
        addTimes := nodeC.Times * nodeD.Times
        times, existed := timesOfSum[-sumOfCD]
        if existed{
            total += times*addTimes
        }
    }
}
return total

}

type GroupNode struct{
Val int
Times int
}

func NewGroup(arr []int) []GroupNode{
sort.Ints(arr)
lastVal := arr[0]-1
nodes := make([]GroupNode, 0)
size := -1

for _, val := range arr{
    if val != lastVal{
        nodes = append(nodes, GroupNode{
            Val: val,
            Times: 0,
        })
        size++
        lastVal = val
    }
    nodes[size].Times++
}

return nodes

}

作者：lonverce
链接：https://leetcode-cn.com/problems/4sum-ii/solution/zhuan-huan-cheng-liang-shu-qiu-he-by-lonverce/
来源：力扣（LeetCode）
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