Beta Round ＃9 (酱油杯noi考后欢乐赛)PLQ的寻宝

题目：http://www.contesthunter.org/contest/Beta%20Round%20%EF%BC%839%20%28%E9%85%B1%E6%B2%B9%E6%9D%AFnoi%E8%80%83%E5%90%8E%E6%AC%A2%E4%B9%90%E8%B5%9B%29/PLQ%E7%9A%84%E5%AF%BB%E5%AE%9D

题解：看了题目觉得分层图SPFA很好想于是就写了，然后A了。

看标程发现也可以DP。。。确实是的

代码：

 #include<cstdio>
 
 #include<cstdlib>
 
 #include<cmath>
 
 #include<cstring>
 
 #include<algorithm>
 
 #include<iostream>
 
 #include<vector>
 
 #include<map>
 
 #include<set>
 
 #include<queue>
 
 #include<string>
 
 #define inf 1000000000
 
 #define maxn 500
 
 #define maxm 500+100
 
 #define eps 1e-10
 
 #define ll long long
 
 #define pa pair<int,int>
 
 #define for0(i,n) for(int i=0;i<=(n);i++)
 
 #define for1(i,n) for(int i=1;i<=(n);i++)
 
 #define for2(i,x,y) for(int i=(x);i<=(y);i++)
 
 #define for3(i,x,y) for(int i=(x);i>=(y);i--)
 
 #define mod 1000000007
 #define sqr(x) (x)*(x)
 
 using namespace std;
 
 inline int read()
 
 {
 
     int x=,f=;char ch=getchar();
 
     while(ch<''||ch>''){if(ch=='-')f=-;ch=getchar();}
 
     while(ch>=''&&ch<=''){x=*x+ch-'';ch=getchar();}
 
     return x*f;
 
 }
 int n,t,k;
 struct rec{double x,y;}a[maxn];
 double c[maxn][maxn],d[maxn][maxn];
 queue<pa>q;
 bool v[maxn][maxn];
 
 int main()
 
 {
 
     freopen("input.txt","r",stdin);
 
     freopen("output.txt","w",stdout);
 
     n=read();t=read();k=read();
     for0(i,n)a[i].x=read(),a[i].y=read();
     for0(i,n)for0(j,n)c[i][j]=sqrt(sqr(a[i].x-a[j].x)+sqr(a[i].y-a[j].y));
     for0(i,n)for0(j,k)d[i][j]=inf;
     d[][]=;
     q.push(pa(,));
     while(!q.empty())
     {
         int x=q.front().first,y=q.front().second;q.pop();
         v[x][y]=;
         for1(i,n)
          if(i!=x&&d[x][y]+c[x][i]<d[i][y+])
          {
              d[i][y+]=d[x][y]+c[x][i];
              if(!v[i][y+]){v[i][y+]=;q.push(pa(i,y+));}
          }
     }
     double ans=inf;
     for1(i,n)if(i!=t)ans=min(ans,d[i][k]+c[i][t]);
     printf("%.2f\n",ans);
 
     return ;
 
 }