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
", 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
", ans );
    }
    return 0;
}