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  • 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
    ",get(1,n,f));
        }
        return 0;
    }
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  • 原文地址:https://www.cnblogs.com/zufezzt/p/5725729.html
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