wikioi 1688--求逆序对

题目描述：给定数组，求逆序对的个数

思路：归并排序，归并的时候改变计数，当前面的元素比后面元素大则计数cnt+=(m-i)+1

没有AC的版本

#include <iostream>
#include <queue>
#include <climits>
#include <algorithm>
#include <memory.h>
#include <stdio.h>
#include <ostream>
#include <vector>
#include <list>
#include <cmath>
#include <string>
#include <stdexcept>
#include <stack>
#include <map>
using namespace std;

vector<int> a;
vector<int> tmpa;
int ans;
void mergeArray(int l,int mid,int r)
{
    int i = l;
    int j = mid+1;
    int k = l;
    while(i <= mid && j <=r)
    {
        if(a[i] > a[j])
        {
            tmpa[k++] = a[j++];
            ans += mid-i+1;
        }
        else
        {
            tmpa[k++] = a[i++];
        }
    }
    while(i <= mid)
        tmpa[k++] = a[i++];
    while(j <= r)
        tmpa[k++] = a[j++];
    for(i = l;i<=r;++i)
        a[i] = tmpa[i];
}
void merge_sort(int l,int r)
{
    if(l<r)
    {
        int mid = (l+r)/2;
        merge_sort(l,mid);
        merge_sort(mid+1,r);
        mergeArray(l,mid,r);
    }
}

int main()
{
    ans = 0;
    int n;
    cin>>n;
    for(int i = 0 ; i < n ; ++i)
    {
        int tmp;
        cin>>tmp;
        a.push_back(tmp);
        tmpa.push_back(tmp);
    }
    merge_sort(0,n-1);
    cout<<ans<<endl;
    return 0;
}