题目1495：关键点

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#include <stdio.h>#include <stdlib.h>#include <string.h>#include <math.h> #define NSIZE 10010#define MSIZE 100010 typedef struct node {    int link ;    int next ;} node ; node data[2*MSIZE] ;int datap ; int links[NSIZE] ; int dis_s[NSIZE] , dis_t[NSIZE] ;int q[NSIZE] , visit[NSIZE] ; int n , m , s , t ; inline int get_new(){    return datap ++ ;} int add_edge( int a , int b ){    int new_id = get_new() ;    data[new_id].link = b ;    data[new_id].next = links[a] ;    links[a] = new_id ;    return 0 ;} int extend_normal( int start , int dis[] ){    for ( int i = 1 ; i <= n ; i ++ ) {        dis[i] = -1 ;    }    dis[start] = 0 ;    int head = 0 ;    int tail = 1 ;    q[head] = start ;    memset( visit , 0 , sizeof(visit) ) ;    visit[start] = 1 ;    while ( tail > head ) {        int cur_node = q[head] ;        int next = links[cur_node] ;        while ( next != -1 ) {            int next_node = data[next].link ;            if ( !visit[next_node] ) {                q[tail] = next_node ;                dis[next_node] = dis[cur_node]+1 ;                visit[next_node] = 1 ;                tail ++ ;            }            next = data[next].next ;        }         head ++ ;    }} int proc_one(){    if ( scanf( "%d%d%d%d" , &n , &m , &s , &t ) == EOF ) {        return 0 ;    }    for ( int i = 1 ; i <= n ; i ++ ) {        links[i] = -1 ;    }    datap = 0 ;    for ( int i = 0 ; i < m ; i ++ ) {        int a , b ;        scanf( "%d%d" , &a , &b ) ;        add_edge( a , b ) ;        add_edge( b , a ) ;    }    extend_normal( s , dis_s ) ;    extend_normal( t , dis_t ) ;    int min_dis = dis_s[t] ; // should be equal to dis_t[s]    if ( min_dis == -1 ) {        printf( "0\n" ) ;        return 1 ;    }     // do final extend    int head = 0 ;     int tail = 1 ;    q[head] = s ;    int disl = 0 ;     int disr = min_dis ;    memset( visit , 0 , sizeof(visit) ) ;    visit[s] = 1 ;    int ans = 0 ;    while ( 1 ) {        int ori_tail = tail ;        for ( int i = head ; i < ori_tail ; i ++ ) {            int cur_node = q[i] ;            int next = links[cur_node] ;            while ( next != -1 ) {                int next_node = data[next].link ;                 if ( visit[next_node] ) {                    next = data[next].next ;                    continue ;                }                if ( dis_s[next_node] != disl + 1 || dis_t[next_node] != disr - 1 ) {                    next = data[next].next ;                    continue ;                }                q[tail] = next_node ;                  visit[next_node] = 1 ;                tail ++ ;                next = data[next].next ;            }        }        if ( visit[t] ) {            break ;        }        head = ori_tail ;        if ( tail - head == 1 ) {            ans ++ ;        }        disl ++ ;        disr -- ;    }    printf( "%d\n" , ans ) ;    return 1 ;} int main(){    while ( proc_one() ) {        ;    }    return 0 ;}/**************************************************************    Problem: 1495    User: cust123    Language: C++    Result: Accepted    Time:190 ms    Memory:2780 kb****************************************************************/
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