P2296 寻找道路

#include <bits/stdc++.h>
using namespace std;
const int maxn = 10005;
set<int> to[maxn];
queue<int> q;int s, t;int n, m;set<int> to2[maxn];
int dist[maxn], used[maxn], dist2[maxn];
int main() {
//    freopen("input.in", "r", stdin);
    cin >> n >> m;
    for(int i = 1; i <= m; i++) {
        int x, y;
        cin >> x >> y;
        to[x].insert(y);
        to2[y].insert(x);
    }
    cin >> s >> t;
    //-----------------
    q.push(t);
    memset(dist2, -1, sizeof(dist2));
    dist2[t] = 0;
    memset(used, 0, sizeof(used));
    used[t] = 1;
    while(!q.empty()) {
        int x = q.front();
        q.pop();
        set<int>::iterator it;
        for(it = to2[x].begin(); it != to2[x].end(); it++) {
            if(!used[*it]) {
                dist2[*it] = dist2[x] + 1;
                used[*it] = 1;
                q.push(*it);
            }
        }
    }
    memset(used, 0, sizeof(used));
    for(int i = 1; i <= n; i++) {
        if(dist2[i] == -1) {
            set<int>::iterator it;
            for(it = to2[i].begin(); it!=to2[i].end(); it++) {
                used[*it] = 1;
            }
        }
    }
    //-----------------
    memset(dist, -1, sizeof(dist));
    dist[s] = 0;
    q.push(s);
    used[s] = 1;
    while(!q.empty()) {
        int x = q.front();
        q.pop();
        set<int>::iterator it;
        for(it = to[x].begin(); it != to[x].end(); it++) {
            if(!used[*it]) {
                dist[*it] = dist[x] + 1;
                used[*it] = 1;
                q.push(*it);
            }
        }
    }
    cout << dist[t];
}