BZOJ-2662: [BeiJing wc2012]冻结（SP

题目：http://www.lydsy.com/JudgeOnline/problem.php?id=2662

裸SPFA。。。没了。。。

代码：

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <deque>
 
using namespace std ;
 
#define AddEdge( s , t , d ) Add( s , t , d ) , Add( t , s , d )
#define pf push_front
#define pb push_back
 
const int inf = 0x7fffffff ;
const int maxn = 50 + 5 ;
const int maxk = 50 + 5 ;
 
struct edge {
    edge *next ;
    int t , d ;
} *head[ maxn ] ;
 
void Add( int s , int t , int d ) {
    edge *p = new( edge ) ;
    p -> t = t , p -> d = d , p -> next = head[ s ] ;
    head[ s ] = p ;
}
 
int dist[ maxn ][ maxk ] , n , m , k ;
 
struct node {
    int x , y ;
    node( int _x , int _y ) : x( _x ) , y( _y ) {
    }
};
deque < node > q ;
 
bool f[ maxn ][ maxk ] ;
 
int main(  ) {
    memset( head , 0 , sizeof( head ) ) ;
    scanf( "%d%d%d" , &n , &m , &k ) ;
    while ( m -- ) {
        int s , t , d ; scanf( "%d%d%d" , &s , &t , &d ) ;
        AddEdge( s , t , d ) ;
    }
    q.clear(  ) ;
    for ( int i = 0 ; i ++ < n ; ) {
        for ( int j = 0 ; j <= k ; ++ j ) {
            dist[ i ][ j ] = inf ; 
        }
    }
    memset( f , false , sizeof( f ) ) ;
    dist[ 1 ][ 0 ] = 0 , q.pf( node( 1 , 0 ) ) , f[ 1 ][ 0 ] = true ;
    while ( ! q.empty(  ) ) {
        node v = q.front(  ) ; q.pop_front(  ) , f[ v.x ][ v.y ] = false ;
        for ( edge *p = head[ v.x ] ; p ; p = p -> next ) {
            if ( dist[ p -> t ][ v.y ] > dist[ v.x ][ v.y ] + p -> d ) {
                dist[ p -> t ][ v.y ] = dist[ v.x ][ v.y ] + p -> d ;
                if ( ! f[ p -> t ][ v.y ] ) {
                    f[ p -> t ][ v.y ] = true ;
                    if ( ! q.empty(  ) && dist[ p -> t ][ v.y ] < dist[ q.front(  ).x ][ q.front(  ).y ] ) {
                        q.pf( node( p -> t , v.y ) ) ;
                    } else q.pb( node( p -> t , v.y ) ) ;
                }
            }
            if ( v.y < k && dist[ p -> t ][ v.y + 1 ] > dist[ v.x ][ v.y ] + p -> d / 2 ) {
                dist[ p -> t ][ v.y + 1 ] = dist[ v.x ][ v.y ] + p -> d / 2 ;
                if ( ! f[ p -> t ][ v.y + 1 ] ) {
                    f[ p -> t ][ v.y ] = true ;
                    if ( ! q.empty(  ) && dist[ q.front(  ).x ][ q.front(  ).y ] > dist[ p -> t ][ v.y + 1 ] ) {
                        q.pf( node( p -> t , v.y + 1 ) ) ;
                    } else q.pb( node( p -> t , v.y + 1 ) ) ;
                }
            }
        }
    }
    int ans = inf ;
    for ( int i = 0 ; i <= k ; ++ i ) ans = min( ans , dist[ n ][ i ] ) ;
    printf( "%d\n" , ans ) ;
    return 0 ;
}