bzoj 1415: [Noi2005]聪聪和可可

直接上记忆化搜索

#include<queue>
#include<cstdio>
#include<algorithm>
using namespace std;
int read_p,read_ca;
inline int read(){
    read_p=0;read_ca=getchar();
    while(read_ca<‘0‘||read_ca>‘9‘) read_ca=getchar();
    while(read_ca>=‘0‘&&read_ca<=‘9‘) read_p=read_p*10+read_ca-48,read_ca=getchar();
    return read_p;
}
struct na{
    int y,ne;
}b[10001];
queue <int> q;
int a,bb;
int n,m,l[3001],r[3001],ru[3001],num=0;
int ne[1001][1001],dis[1001][1001];
double jy[1001][1001];
bool bo[1001][1001];
const int INF=1e9;
inline void add(int x,int y){
    num++;
    if (!l[x]) l[x]=num;else b[r[x]].ne=num;
    b[num].y=y;r[x]=num;
}
inline void bfs(int x){
    for (int i=1;i<=n;i++) dis[x][i]=INF;
    dis[x][x]=0;ne[x][x]=x;
    for (int i=l[x];i;i=b[i].ne) dis[x][b[i].y]=1,ne[x][b[i].y]=b[i].y,q.push(b[i].y);
    while (!q.empty()){
        int k=q.front();q.pop();
        for (int i=l[k];i;i=b[i].ne)
        if (dis[x][b[i].y]>dis[x][k]+1) dis[x][b[i].y]=dis[x][k]+1,ne[x][b[i].y]=ne[x][k],q.push(b[i].y);else
        if (dis[x][b[i].y]==dis[x][k]+1&&ne[x][b[i].y]>ne[x][k]) ne[x][b[i].y]=ne[x][k],q.push(b[i].y);
    }
}
inline double dfs(int a,int bb){
    if (a==bb) return 0.0;
    if (bo[a][bb]) return jy[a][bb];
    bo[a][bb]=1;
    double ans=0.0;
    int aa=ne[ne[a][bb]][bb];
    if (aa==bb) return jy[a][bb]=1.0;
    int u=1;
    for (int i=l[bb];i;i=b[i].ne) ans+=dfs(aa,b[i].y),u++;
    ans+=dfs(aa,bb);
    ans=1.0/u*ans+1.0;
    return jy[a][bb]=ans;
}
int main(){
    int i,j,x,y;
    n=read();m=read();
    a=read();bb=read();
    for (i=1;i<=m;i++) x=read(),y=read(),add(x,y),add(y,x);
    for (int i=1;i<=n;i++) bfs(i);
    printf("%.3lf\n",dfs(a,bb));
    return 0;
}