题目：https://www.lydsy.com/JudgeOnline/problem.php?id=1415
看博客：http://www.cnblogs.com/Narh/p/9206642.html
看博客：https://blog.csdn.net/clove_unique/article/details/62237321
代码如下：
#include<iostream> #include<cstdio> #include<cstring> #include<queue> using namespace std; int const maxn=1005; int n,m,cc,kk,head[maxn],ct,dis[maxn][maxn],goal[maxn][maxn],inf; double d[maxn],f[maxn][maxn],ans; queue<int>q; struct N{ int to,next; N(int t=0,int n=0):to(t),next(n) {} }edge[maxn<<1]; void add(int x,int y){edge[++ct]=N(y,head[x]); head[x]=ct;} void bfs(int s) { while(q.size())q.pop(); dis[s][s]=0; q.push(s); while(q.size()) { int x=q.front(); q.pop(); for(int i=head[x];i;i=edge[i].next) { int u=edge[i].to; if(dis[s][u]==inf) { dis[s][u]=dis[s][x]+1; q.push(u); } } } } double dfs(int c,int k) { if(f[c][k]>=0)return f[c][k]; if(c==k)return f[c][k]=0; if(dis[c][k]<=2)return f[c][k]=1; double P=1.0/(d[k]+1.0); f[c][k]=1;// int to=goal[c][k]; for(int i=head[k];i;i=edge[i].next) f[c][k]+=P*dfs(to,edge[i].to); f[c][k]+=P*dfs(to,k); return f[c][k]; } int main() { scanf("%d%d%d%d",&n,&m,&cc,&kk); for(int i=1,x,y;i<=m;i++) { scanf("%d%d",&x,&y); add(x,y); add(y,x); d[x]+=1.0; d[y]+=1.0; } memset(dis,0x3f,sizeof dis); inf=dis[0][0]; for(int i=1;i<=n;i++)bfs(i); for(int i=1;i<=n;i++)//cc for(int j=1;j<=n;j++)//kk { if(dis[i][j]==inf)continue; if(dis[i][j]<=2){goal[i][j]=j; continue;} int mx=inf,t=inf; for(int k=head[i];k;k=edge[k].next) { int u=edge[k].to; if(dis[u][j]<mx || (dis[u][j]==mx&&u<t))mx=dis[u][j],t=u; } int mx2=inf,t2=inf; for(int k=head[t];k;k=edge[k].next) { int u=edge[k].to; if(dis[u][j]<mx2 || (dis[u][j]==mx2&&u<t2))mx2=dis[u][j],t2=u; } goal[i][j]=t2; } memset(f,-3,sizeof f); ans=dfs(cc,kk); printf("%.3lf",ans); return 0; }