Mathematics:Ultra-QuickSort(POJ 2299)

　　　　　　　　　　　　　　　　　　

　　　　　　　　　　　　　　　　　　极度快速排序

　　题目大意：在一个输入数组中找逆序数。。。

　　水题，求逆序数的很好的算法，就是MergeSort，和我之前发的DNA那个差不多，最后就是后台数据很大，答案要用long long

　　

#include <iostream>
#include <functional>
#include <algorithm>

using namespace std;

static int num[500005], tmp[500005];
typedef int Position;

void Merge_Sort(Position, Position, long long *const);
void Merge(Position,Position, Position, long long *const);

int main(void)
{
    int length;
    long long ans;
    while (~scanf("%d", &length))
    {
        if (length == 0)break;
        for (int i = 0; i < length; i++)
            scanf("%d", &num[i]);
        ans = 0;
        Merge_Sort(0, length - 1, &ans);
        printf("%lld
", ans);
    }
    return 0;
}

void Merge_Sort(Position Left, Position Right, long long *const reverse_num)
{
    if (Left < Right)
    {
        Position Mid = (Left + Right) / 2;
        Merge_Sort(Left, Mid, reverse_num);
        Merge_Sort(Mid + 1, Right, reverse_num);
        Merge(Left, Mid, Right, reverse_num);
    }
}

void Merge(Position Left, Position mid, Position Right, long long *const reverse_num)
{
    Position lpos = Left, rpos = mid + 1, lend = mid, rend = Right, pos = Left;

    while (lpos <= lend && rpos <= rend)
    {
        if (num[lpos] <= num[rpos])
            tmp[pos++] = num[lpos++];
        else
        {
            (*reverse_num) += lend - lpos + 1;
            tmp[pos++] = num[rpos++];
        }
    }
    while (lpos <= lend)
        tmp[pos++] = num[lpos++];
    while (rpos<=rend)
        tmp[pos++] = num[rpos++];
    for (pos = Left; pos <= Right; pos++)
        num[pos] = tmp[pos];
}