HDU4911——归并排序——Inversion

http://acm.hdu.edu.cn/showproblem.php?pid=4911

/*
裸题
有重复的不能用树状数组！！！！sort的时候会出错
*/
/************************************************
* Author        :Powatr
* Created Time  :2015-8-19 16:24:11
* File Name     :A.cpp
 ************************************************/

#include <cstdio>
#include <algorithm>
#include <iostream>
#include <sstream>
#include <cstring>
#include <cmath>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <bitset>
#include <cstdlib>
#include <ctime>
using namespace std;

#define lson l, mid, rt << 1
#define rson mid + 1, r, rt << 1 | 1
typedef long long ll;
const int MAXN = 1e5 + 10;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;

int a[MAXN],temp[MAXN];
ll ans = 0;
void mergesort(int *a,int  first,int  mid,int  last,int *temp)
{
   int i = first,j = mid + 1;
   int  k = first;
    while( i <= mid && j <= last){
          if(a[i] <= a[j])
          temp[k++] = a[i++];
          else {
            ans += j - k;
           temp[k++] = a[j++];
          }
    }
    while( i <= mid)
        temp[k++] = a[i++];
    while( j <= last)
        temp[k++] = a[j++];
    for(int i = first;i <= last;i++)
        a[i] = temp[i];
}
void mergearray(int *a,int first,int last,int *temp)
{
     if(first < last){
            int mid = (first + last) >> 1;
            mergearray(a,first,mid,temp);
            mergearray(a,mid+1,last,temp);
            mergesort(a,first,mid,last,temp);
     }
}
int main()
{
    int n,m;
    while(~ scanf("%d%d",&n, &m)){
        ans = 0;
        memset(temp, 0, sizeof(temp));
    for(int i = 1; i <= n; i++)
    scanf("%d",&a[i]);
    mergearray(a,1,n,temp);
     ans -= m;
    if(ans < 0) ans = 0;
    printf("%I64d\n",ans);
    }
  return 0;
}