Crossing Rivers uva12230

https://www.luogu.com.cn/problem/UVA12230

期望的线性性质

设初始答案 A，为全走陆地的时间 D，则每次输入时去河的长度 L，加上渡河期望时间 2* L/v

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#include<iostream> #include<cstring> #include<algorithm> #include<set> using namespace std;  int n,D;  signed main() {     int cas=0;     while(cin>>n>>D,n||D){         double ans=D;         for(int i=1;i<=n;i++){             int p,L,v;             cin>>p>>L>>v;             ans-=L;             ans+=(double)2*L/v;         }         printf("Case %d: %.3lf\n\n",++cas,ans);     }  }