51Nod 1649 齐头并进

#include <iostream>
#include <algorithm>
#include <cstring>
//两遍迪杰斯特拉
#define INF 0xfffffff
using namespace std;
const int maxn = 400 + 5;
int rail[maxn][maxn];
int dis[maxn];
int vis[maxn];

void dij(int n, int u){
    int i, j, p, Min;
    memset(vis, 0, sizeof(vis));
    for (int i = 1; i <= n; i++)
        dis[i] = rail[u][i];
    vis[u] = 1;
    dis[u] = 0;

    for (i = 1; i < n; i++){
        Min = INF;
        for (j = 1; j <= n; j++){
            if (!vis[j] && dis[j] < Min){
                p = j;
                Min = dis[j];
            }
        }
        if (Min == INF) return;
        vis[p] = 1;
        for (j = 1; j <= n; j++){
            if (!vis[j] && dis[p] + rail[p][j] < dis[j]){
                dis[j] = dis[p] + rail[p][j];
            }
        }
    }
}

int main(){
    ios::sync_with_stdio(false);
    fill(rail[0], rail[0] + maxn*maxn, INF);
    int n, m;
    cin >> n >> m;
    if (m == 0){
        //没有铁路
        cout << "-1" << endl;
        return 0;
    }
    for (int i = 0; i < m; i++){
        int x, y;
        cin >> x >> y;
        rail[x][y] = 1;
        rail[y][x] = 1;
    }
    dij(n, 1);
    int ans1 = dis[n];
    if (ans1 == INF || m == (n*(n - 1) / 2)){
        //铁路没有答案，或者没有公路
        cout << "-1" << endl;
        return 0;
    }

    for (int i = 1; i <= n; i++){
        for (int j = 1 + i; j <= n; j++){
            if (rail[i][j] == 1){
                rail[i][j] = INF;
                rail[j][i] = INF;
            }
            else{
                rail[i][j] = 1;
                rail[j][i] = 1;
            }
        }
    }
    dij(n, 1);
    int ans2 = dis[n];
    int ans = ans1 > ans2 ? ans1 : ans2;
    cout << ans << endl;


    system("pause");
    return 0;
}