1415: [Noi2005]聪聪和可可

%:pragma GCC optimize(3)
#include<bits/stdc++.h>
using namespace std;

const int N = 1005;
const int M = N * 5;

int pos

, dis

, n , m , to[M] , ne[M] , cnt , fir
, du
, s  , t , x , y , pre
;
double dp

;
bool vis

;

#define Foreachson(i,x) for(int i = fir[x];i;i = ne[i])

void add(int x ,int y) {
ne[++ cnt] = fir[x]; fir[x] = cnt; to[cnt] = y;
}

void link(int x ,int y) {
add(x,y); add(y,x);
du[x] ++; du[y] ++;
}

void BFS(int x) {
queue<int>q;
dis[x][x] = 0;
memset(pre,0,sizeof(pre));
Foreachson(i,x) {
int V = to[i];
dis[x][V] = 1;
pre[V] = V;
q.push(V);
}
while(!q.empty()) {
int ind = q.front(); q.pop();
Foreachson(i,ind) {
int V = to[i];
if(dis[x][V] > dis[x][ind] + 1) {
dis[x][V] = dis[x][ind] + 1;
pre[V] = pre[ind];
q.push(V);
}
else if(dis[x][V] == dis[x][ind] + 1) {
if(pre[V] > pre[ind]) {
pre[V] = pre[ind];
q.push(V);
}
}
}
}

for(int i = 1;i <= n;i ++) {
pos[x][i] = pre[i];
}
pos[x][x] = x;
}

double dfs(int x ,int y) {
if(vis[x][y]) return dp[x][y];
//  vis[x][y] = 1;
if(x == y) return 0;
if(dis[x][y] <= 2) return 1;
vis[x][y] = 1;
Foreachson(i,y) {
int V = to[i];
dp[x][y] = dp[x][y] + (double)1 / (double)(du[y] + 1) * dfs(pos[pos[x][y]][y] , V);
}
int V = y;
dp[x][y] += (double)1 / (double)(du[y] + 1) * (dfs(pos[pos[x][y]][y] , V));
dp[x][y] += 1.0;
return dp[x][y];
}

int main(){
scanf("%d%d",&n,&m);
scanf("%d%d",&s,&t);
for(int i = 1;i <= m;i ++) {
scanf("%d%d",&x,&y);
link(x,y);
}

memset(dis,127,sizeof(dis));

for(int i = 1;i <= n;i ++) BFS(i);
printf("%.3lf",dfs(s,t));
}