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=0,f=1;char ch=getchar();

    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}

    while(ch>='0'&&ch<='9'){x=10*x+ch-'0';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[0][0]=0;
    q.push(pa(0,0));
    while(!q.empty())
    {
        int x=q.front().first,y=q.front().second;q.pop();
        v[x][y]=0;
        for1(i,n)
         if(i!=x&&d[x][y]+c[x][i]<d[i][y+1])
         {
             d[i][y+1]=d[x][y]+c[x][i];
             if(!v[i][y+1]){v[i][y+1]=1;q.push(pa(i,y+1));}
         }
    }
    double ans=inf;
    for1(i,n)if(i!=t)ans=min(ans,d[i][k]+c[i][t]);
    printf("%.2f
",ans);

    return 0;

}