HDU 3743 Frosh Week

　　归并排序求逆序数

#include <iostream>
#include <cstdio>
using namespace std;
#define maxn 1000005
int a[maxn], temp[maxn];
long long ans;
void MergeSort(int a[], int l, int mid, int r)
{
    int k=0;
    int i = l, n = mid, j = mid, m = r;
    while ( i<n && j<m )
    {
        if (a[i] <= a[j])
        {
            temp[k++] = a[i++];
        }
        else
        {
            ans += n-i;
            temp[k++] = a[j++];
        }
    }
    while (i<n)
        temp[k++] = a[i++];
    while (j<m)
        temp[k++] = a[j++];
    for (int t = 0; t<k; ++t)
        a[l+t] = temp[t];
}
void Sort(int a[], int l, int r)
{
    if (r-l<=1)
        return ;
    int mid = (l+r)>>1;
    Sort(a, l, mid);
    Sort(a, mid, r);
    MergeSort(a, l, mid, r);
}

int main()
{
    int n;
    while (~scanf("%d", &n))
    {
        for (int i=0; i < n; ++i)
            scanf("%d", &a[i]);
        ans = 0;
        Sort(a, 0, n);
        printf("%lld\n", ans);
    }
    return 0;
}