51nod 1649 齐头并进 (dijkstra)

两次最短路找最大值；

#pragma GCC optimize(2)

#include <bits/stdc++.h>

#define maxn 405

typedef long long ll;

using namespace std;

ll a[maxn][maxn];
ll b[maxn][maxn];
ll mo = 1e18;
ll dis[maxn];
ll flag[maxn],mi,n,k;
void dfs1(ll be)
{
    for(int i = 1; i <= n; i ++)
    {
        dis[i] = 1e18;
    }
    dis[be] = 0;
    //flag[be] = 1;
    for(int i = 1; i <= n; i ++)
    {
        mi = mo;
        for(int j = 1; j <= n; j ++)
        {
            if(!flag[j] && dis[j] < mi)
            {
                mi = dis[j];
                k = j;
            }
        }
        if(mi == mo)break;
        flag[k] = 1;
        for(int j = 1; j <= n; j ++)
        {
            if(!flag[j] && dis[j] > dis[k] + a[k][j])
            {
                dis[j] = dis[k] + a[k][j];
            }
        }
    }
}

void dfs2(ll be)
{
    for(int i = 1; i <= n; i ++)
    {
        dis[i] = 1e18;
    }
    dis[be] = 0;
    //flag[be] = 1;
    for(int i = 1; i <= n; i ++)
    {
        mi = mo;
        for(int j = 1; j <= n; j ++)
        {
            if(!flag[j] && dis[j] < mi)
            {
                mi = dis[j];
                k = j;
            }
        }
        if(mi == mo)break;
        flag[k] = 1;
        for(int j = 1; j <= n; j ++)
        {
            if(!flag[j] && dis[j] > dis[k] + b[k][j])
            {
                dis[j] = dis[k] + b[k][j];
            }
        }
    }
}
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);

    ll m;
    cin >> n >> m;
    for(int i = 1; i <= n; i ++)
    {
        for(int j = 1; j <= n ; j ++)
        {
            a[i][j] = 1e18;
        }
    }
    for(int i = 1; i <= m ; i ++)
    {
        ll x,y;
        cin >> x >> y;
        a[x][y] = 1;
        a[y][x] = 1;
    }

    for(int i = 1; i <= n; i ++)
    {
        for(int j = 1; j <= n ; j ++)
        {
            b[i][j] = 1e18;
        }
    }
    for(int i = 1; i <= n; i ++)
    {
        for(int j = 1; j <= n; j ++)
        {
            if(i != j && a[i][j] == mo)
            {
                b[i][j] = 1;
                b[j][i] = 1;
            }
        }
    }

    dfs1(1);
    ll m1 = dis[n];
    if(m1 == mo)cout << -1 << endl;
    else
    {
        memset(flag,0,sizeof(flag));
        dfs2(1);
        k = 0;
        ll m2 = dis[n];
        if(m2 == mo)cout << -1 << endl;
        else
        {
            cout << max(m1,m2) << endl;
        }
    }
    return 0;
}