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：
1 #include<cstdio> 2 3 #include<cstdlib> 4 5 #include<cmath> 6 7 #include<cstring> 8 9 #include<algorithm> 10 11 #include<iostream> 12 13 #include<vector> 14 15 #include<map> 16 17 #include<set> 18 19 #include<queue> 20 21 #include<string> 22 23 #define inf 1000000000 24 25 #define maxn 500 26 27 #define maxm 500+100 28 29 #define eps 1e-10 30 31 #define ll long long 32 33 #define pa pair<int,int> 34 35 #define for0(i,n) for(int i=0;i<=(n);i++) 36 37 #define for1(i,n) for(int i=1;i<=(n);i++) 38 39 #define for2(i,x,y) for(int i=(x);i<=(y);i++) 40 41 #define for3(i,x,y) for(int i=(x);i>=(y);i--) 42 43 #define mod 1000000007 44 #define sqr(x) (x)*(x) 45 46 using namespace std; 47 48 inline int read() 49 50 { 51 52 int x=0,f=1;char ch=getchar(); 53 54 while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();} 55 56 while(ch>='0'&&ch<='9'){x=10*x+ch-'0';ch=getchar();} 57 58 return x*f; 59 60 } 61 int n,t,k; 62 struct rec{double x,y;}a[maxn]; 63 double c[maxn][maxn],d[maxn][maxn]; 64 queue<pa>q; 65 bool v[maxn][maxn]; 66 67 int main() 68 69 { 70 71 freopen("input.txt","r",stdin); 72 73 freopen("output.txt","w",stdout); 74 75 n=read();t=read();k=read(); 76 for0(i,n)a[i].x=read(),a[i].y=read(); 77 for0(i,n)for0(j,n)c[i][j]=sqrt(sqr(a[i].x-a[j].x)+sqr(a[i].y-a[j].y)); 78 for0(i,n)for0(j,k)d[i][j]=inf; 79 d[0][0]=0; 80 q.push(pa(0,0)); 81 while(!q.empty()) 82 { 83 int x=q.front().first,y=q.front().second;q.pop(); 84 v[x][y]=0; 85 for1(i,n) 86 if(i!=x&&d[x][y]+c[x][i]<d[i][y+1]) 87 { 88 d[i][y+1]=d[x][y]+c[x][i]; 89 if(!v[i][y+1]){v[i][y+1]=1;q.push(pa(i,y+1));} 90 } 91 } 92 double ans=inf; 93 for1(i,n)if(i!=t)ans=min(ans,d[i][k]+c[i][t]); 94 printf("%.2f ",ans); 95 96 return 0; 97 98 }