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;
}

转载于:https://www.cnblogs.com/GBRgbr/p/3361354.html