HDU 2689 Sort it

求逆序数。分治写法。

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-8;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
inline int read()
{
    char c = getchar();  while(!isdigit(c)) c = getchar();
    int x = 0;
    while(isdigit(c)) { x = x * 10 + c - '0'; c = getchar(); }
    return x;
}

int n,a[1010];

int get(int l,int r,vector<int> &b)
{
    if(l==r) { b.push_back(a[l]); return 0; }
    int m=(l+r)/2; vector<int>L,R;
    int x1=get(l,m,L), x2=get(m+1,r,R);
    int x3=0,pl=0,pr=0;
    for(;;)
    {
        if(pl==L.size()&&pr==R.size()) break;
        if(pl<L.size()&&pr<R.size())
        {
            if(L[pl]<R[pr]) b.push_back(L[pl++]);
            else b.push_back(R[pr++]), x3=x3+L.size()-pl;
        }
        else if(pl<L.size()&&pr==R.size()) b.push_back(L[pl++]);
        else if(pl==L.size()&&pr<R.size()) b.push_back(R[pr++]), x3=x3+L.size()-pl;
    }
    return x1+x2+x3;
}

int main()
{
    while(~scanf("%d",&n))
    {
        for(int i=1;i<=n;i++) scanf("%d",&a[i]);
        vector<int> f; printf("%d\n",get(1,n,f));
    }
    return 0;
}