HDU 4460 Friend Chains ( BFS )

每个点求一次最短路，在最短路中找最长的。

用邻接表存边，尽量不要用STL，很容易超时。

#include <cstdio>

#include <cstring>

#include <cstdlib>

#include <algorithm>



using namespace std;



const int MAXN = 1010;

const int INF  = 1 << 22;



struct node

{

    int v, next;

    node() { }

    node( int v, int next ):v(v), next(next) { }

};



struct Dijkstra

{

    int n, m;

    int head[MAXN];

    node edges[MAXN*22];

    bool done[MAXN];

    int d[MAXN];



    void init( int n )

    {

        this->n = n;

        this->m = 0;

        for ( int i = 0; i <= n; ++i ) head[i] = -1;

        return;

    }



    void AddEdge( int from, int to )

    {

        edges[m].v = to;

        edges[m].next = head[from];

        head[from] = m++;

        return;

    }



    void dijkstra( int s )

    {

        int Q[MAXN*22];

        for ( int i = 0; i <= n; ++i ) d[i] = INF;

        d[s] = 0;

        memset( done, false, sizeof(done) );



        int front = 0;

        int tail = 0;

        Q[tail++] = s;



        while ( front < tail )

        {

            int u = Q[front++];

            if ( done[u] ) continue;

            done[u] = true;

            for ( int i = head[u]; i != -1; i = edges[i].next )

            {

                int v = edges[i].v;

                if ( d[v] > d[u] + 1 )

                {

                    d[v] = d[u] + 1;

                    Q[tail++] = v;

                }

            }

            //printf( "front=%d tail=%d\n", front, tail );

        }

        return;

    }

};



int N, edgeN;

Dijkstra slv;

char name[MAXN][14];



int GetNum( char *s )

{

    for ( int i = 1; i <= N; ++i )

    {

        if ( !strcmp( s, name[i] ) ) return i;

    }

    return 0;

}



int main()

{

    while ( scanf("%d", &N ) == 1 && N )

    {

        for ( int i = 1; i <= N; ++i )

            scanf( "%s", name[i] );

        scanf( "%d", &edgeN );

        slv.init( N );



        for ( int i = 0; i < edgeN; ++i )

        {

            char a[14], b[14];

            scanf( "%s%s", a, b );

            int u = GetNum(a);

            int v = GetNum(b);

            slv.AddEdge( u, v );

            slv.AddEdge( v, u );

        }



        int ans = 0;

        for ( int i = 1; i <= N; ++i )

        {

            slv.dijkstra( i );

            for ( int j = i + 1; j <= N; ++j )

            {

                ans = max( ans, slv.d[j] );

                if ( ans >= INF ) break;

            }

            if ( ans >= INF ) break;

        }

        if ( ans >= INF ) puts("-1");

        else printf( "%d\n", ans );

    }

    return 0;

}