题目分析:考察对floyd的理解,如果存在一条路i->k,k->j,那么如果中间的k是未被标记的,那么我们就不用k去更新i->j之间的最短路,那么除非存在另一条路i->l,l->j且l已经被标记或者存在i->j的边,否则i->j是不可达的。每次标记一个点的时候就用这个点去更新所有的最短路即可。不要忘记重边的判断。
代码如下:
#include <cstdio> #include <cstring> #include <algorithm> using namespace std ; #define REP( i , a , b ) for ( int i = ( a ) ; i < ( b ) ; ++ i ) #define FOR( i , a , b ) for ( int i = ( a ) ; i <= ( b ) ; ++ i ) #define REV( i , a , b ) for ( int i = ( a ) ; i >= ( b ) ; -- i ) #define travel( e , H , u ) for ( Edge* e = H[u] ; e ; e = e -> next ) #define CLR( a , x ) memset ( a , x , sizeof a ) typedef long long LL ; const int MAXN = 300 ; const int INF = 0x3f3f3f3f ; bool vis[MAXN] ; int G[MAXN][MAXN] ; int n , m , q ; void scanf ( int& x , char c = 0 ) { while ( ( c = getchar () ) < '0' || c > '9' ) ; x = c - '0' ; while ( ( c = getchar () ) >= '0' && c <= '9' ) x = x * 10 + c - '0' ; } void solve () { int u , v , c , tmp ; CLR ( G , INF ) ; CLR ( vis , 0 ) ; while ( m -- ) { scanf ( u ) , scanf ( v ) , scanf ( c ) ; if ( G[u][v] > c ) G[u][v] = c ; } while ( q -- ) { scanf ( c ) ; if ( c == 0 ) { scanf ( u ) ; if ( vis[u] ) printf ( "ERROR! At point %d\n" , u ) ; else { vis[u] = 1 ; G[u][u] = 0 ; REP ( i , 0 , n ) REP ( j , 0 , n ) if ( G[i][j] > G[i][u] + G[u][j] ) G[i][j] = G[i][u] + G[u][j] ; } } else { scanf ( u ) , scanf ( v ) ; if ( !vis[u] || !vis[v] ) printf ( "ERROR! At path %d to %d\n" , u , v ) ; else if ( G[u][v] == INF ) printf ( "No such path\n" ) ; else printf ( "%d\n" , G[u][v] ) ; } } } int main () { int cas = 0 ; while ( ~scanf ( "%d%d%d" , &n , &m , &q ) && ( n || m || q ) ) { if ( cas ) printf ( "\n" ) ; printf ( "Case %d:\n" , ++ cas ) ; solve () ; } return 0 ; }