HDU 3743 Frosh Week(归并排序求逆序对)

题目链接:http://acm.hdu.edu.cn/showproblem.php?pid=3743

  题目意思就是给你一个长为n的序列,让你求逆序对.我用的是归并排序来求的.归并排序有一个合并的过程,分前后两段,当a[i] > a[j]时,说明a[j]比前面那段啊[i],a[i+1],a[i+2]....,a[mid],比这些都要小,所以总逆序对数要加上mid-i+1.

// File Name: HDU3743.cpp
// Author: xiaxiaosheng
// Created Time: 2015年03月09日 星期一 21时54分45秒

#include<vector>
#include<list>
#include<map>
#include<set>
#include<deque>
#include<stack>
#include<bitset>
#include<algorithm>
#include<functional>
#include<numeric>
#include<utility>
#include<sstream>
#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<ctime>

using namespace std;
typedef __int64 INT;
const int maxn = 10e6+10;

int n,A[maxn],temp[maxn];
INT ans;
void merger(int *A,int s,int mid,int e)
{
    int k = 0,i = s,j = mid+1;
    while(i <= mid && j <= e)
    {
        if(A[i] < A[j])
            temp[k++] = A[i++];
        else
        {
            ans += (mid-i+1);
            temp[k++] = A[j++];
        }
    }
    while(i <= mid) temp[k++] = A[i++];
    while(j <= e) temp[k++] = A[j++];
    for(int i = 0;i < k;++i)
        A[s+i] = temp[i];
}
void mergersort(int *A,int s,int e)
{
    if(s < e)
    {
        int mid = (s + e) / 2;
        mergersort(A,s,mid);
        mergersort(A,mid+1,e);
        merger(A,s,mid,e);
    }

}
int main(){
    while(scanf("%d",&n)!=EOF)
    {
        for(int i = 0;i < n;++i)
            scanf("%d",&A[i]);
        ans = 0;
        mergersort(A,0,n-1);
        printf("%I64d
",ans);
    }
    return 0;
}