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+;

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

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