NOIp2014 寻找道路

这道题本身真的不怎么难，但是

"路径上所有的点的出边所指向的点都直接或间接与终点连通"

这句话真的是太ex了。。

我隐隐约约记得当年做这道题时的懵逼，看来当时的代码确实是没什么问题，只是因为理解上的误差

教训：好好看题，看明白了再做，要不然一切都是白干。

题干

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
#include<algorithm>

using namespace std;

typedef long long ll;

const int Maxn = 1e4+10,Maxm = 2e5+10;

ll read(){
    ll ans = 0;
    char last = ' ',ch = getchar();
    while(ch < '0'||ch > '9')last = ch,ch = getchar();
    while('0' <= ch&&ch <= '9')ans = ans*10+ch-'0',ch = getchar();
    if(last == '-')return -ans;return ans;
}

struct Edge{
    int to,ne;
}edges[2][Maxm],e;

struct Node{
    int to,d;
    bool operator <(const Node& x)const{
        return d > x.d;
    }
};

int first[2][Maxn];
int d[Maxn],vis[Maxn],v2[Maxn];
int cnte = 0,n,m,u,v,s,t;

void add_edge(int fr,int to){
    edges[1][++cnte] = (Edge){to,first[1][fr]};
    edges[0][cnte] = (Edge){fr,first[0][to]};
    first[1][fr] = first[0][to] = cnte;
}

void dfs(int s){
    vis[s] = 1;
    for(int i = first[0][s];i;i = edges[0][i].ne){
        e = edges[0][i];
        if(vis[e.to])continue;
        dfs(e.to);
    }
}

void del(int s){
    vis[s] = 0;
    for(int i = first[0][s];i;i = edges[0][i].ne){
        e = edges[0][i];
        if(vis[e.to])del(e.to);
    }
}

void dijkstra(int s){
    priority_queue<Node> q;
    q.push((Node){s,0});
    memset(d,0x3f,sizeof(d));
    d[s] = 0;
    while(!q.empty()){
        u = q.top().to;
        q.pop();
        if(vis[u])continue;
        vis[u] = 1;
        for(int i = first[1][u];i;i = edges[1][i].ne){
            e = edges[1][i];
            if(d[e.to] > d[u]+1){
                d[e.to] = d[u]+1;
                q.push((Node){e.to,d[e.to]});
            }
        }
    }
}

int main(){
    n = read(),m = read();
    for(int i = 1;i <= m;i++){
        u = read(),v = read();
        if(u != v)add_edge(u,v);
    }
    s = read(),t = read();
    dfs(t);
    for(int i = 1;i <= n;i++)if(!vis[i]){
        v2[i] = 1;
        for(int j = first[0][i];j;j = edges[0][j].ne){
            e = edges[0][j];
            v2[e.to] = 1;
        }
    }
    for(int i = 1;i <= n;i++)vis[i] = v2[i];
    vis[s] = 0; 
    dijkstra(s);
    if(d[t] == d[0])cout << -1 << '
';
    else cout << d[t] << '
';
return 0;
}