codevs3731 寻找道路

方向dfs判定是否可行，spfa跑最短路。

noip水题，wa好几次。

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn = 10000 + 10;
const int maxm = 200000 + 10;

int g[maxn],v[maxm],next[maxm],eid;
int g1[maxn],v1[maxm],next1[maxm],eid1;
int d[maxn],q[maxn*10];
int n,m,S,T;
bool vis[maxn],able[maxn],ok[maxn],inque[maxn];

void addedge(int a,int b) {
    v[eid]=b; next[eid]=g[a]; g[a]=eid++;
}

void add(int a,int b) {
    v1[eid1]=b; next1[eid1]=g1[a]; g1[a]=eid1++;
}

void dfs(int u) {
    vis[u]=ok[u]=1;
    for(int i=g1[u];~i;i=next1[i])
        if(!vis[v1[i]]) dfs(v1[i]);
}

void spfa() {
    int l=0,r=0,u;
    memset(d,0x3f,sizeof(d));
    d[S]=0;
    q[r++]=S;
    while(l<r) {
        u=q[l++];
        for(int i=g[u];~i;i=next[i]) if(able[v[i]] && d[v[i]] > d[u]+1) {
            d[v[i]]=d[u]+1;
            if(!inque[v[i]]) {
                q[r++]=v[i];
                inque[v[i]]=1;
            }
        }
        inque[u]=0;
    }
}

int main() {
    memset(g,-1,sizeof(g));
    memset(g1,-1,sizeof(g1));
    scanf("%d%d",&n,&m);
    for(int i=1,x,y;i<=m;i++) {
        scanf("%d%d",&x,&y);
        if(x!=y) {
            addedge(x,y);
            add(y,x);
        }
    }

    scanf("%d%d",&S,&T);
    memset(able,1,sizeof(able));
    dfs(T);

    for(int u=1;u<=n;u++) {
        for(int i=g[u];~i;i=next[i]) if(!ok[v[i]]) {
            able[u]=0;
            break;
        }
    }

    spfa();
    if(d[T]>=n) printf("-1\n");
    else printf("%d\n",d[T]);
    return 0;
}