题解 | #【模板】单源最短路1#

#include <climits>
#include <iostream>
#include <queue>
#include <vector>
using namespace std;

int main() {
    int n = 0, m = 0, u = 0, v = 0;
    cin >> n >> m;
    n--;
    vector<queue<int>> table(5000, queue<int>());
    vector<int> dist(5000, INT_MAX);
    vector<int> flag(5000, 0);
    for (int i = 0; i < m; i++) {
        cin >> u >> v;
        u--;
        v--;
        table[u].push(v);
        table[v].push(u);
        if (!u) {
            dist[v] = 1;
            flag[v] = 1;
        }
        if (!v) {
            dist[u] = 1;
            flag[u] = 1;
        }
    }
    dist[0] = 0;
    flag[0] = 1;
    int k = 5000;
    if (k > m) k = m;
    while (k--&&!flag[n]) {
        for (int i = 1; i < 5000; i++) {
            if (flag[i]) {
                while (!table[i].empty()) {
                    if (dist[i] + 1 < dist[table[i].front()]) dist[table[i].front()] = dist[i] + 1;
                    table[i].pop();
                }
            }
        }
        for (int i = 0; i < 5000; i++) {
            if (dist[i] < INT_MAX) flag[i] = 1;
        }
    }
    if (flag[n]) cout << dist[n] << endl;
    else cout << -1 << endl;
    return 0;
}
// 64 位输出请用 printf("%lld")