逆序对的数量-golang

逆序对的数量-golang

package main

import "fmt"

var n int

var arr []int

func main(){
    fmt.Scanf("%d", &n)

    arr = make([]int, n)

    for i := 0; i <n; i++{
        fmt.Scanf("%d", &arr[i])
    }

    var num int
    mergeSort(arr, 0, n-1, &num)

    fmt.Println(num)
}

func mergeSort(arr []int, l, r int, num *int){
    if l >= r{
        return 
    }
    mid := (l+r)>>1

    mergeSort(arr, l, mid, num)
    mergeSort(arr, mid+1, r, num)

    tmp := make([]int, 0, r-l+1)

    i, j := l, mid+1


    for i <= mid && j <= r{
        if arr[i] <= arr[j]{
            tmp = append(tmp, arr[i])
            i++
            continue
        }
        tmp = append(tmp, arr[j])
        *num+= mid-i+1
        j++
    }

    for i <= mid{
        tmp = append(tmp, arr[i])
        i++
    }

    for j <= r{
        tmp = append(tmp, arr[j])
        j++
    }
    copy(arr[l:r+1], tmp)
}