久违的bfs

#include<bits/stdc++.h>
using namespace std;
bool vis[ 100001 ] ;
int step[ 100001 ] ;
int n , k ;
queue<int> q ;
int main()
{
    memset( step , -1 , sizeof( step ) ) ;
    ios::sync_with_stdio( 0 ) ;
    cin.tie( 0 ) ;
    cin >> n >> k ;
    q.push( n ) ;
    step[ n ] = 0 ;
    vis[ n ] = 1 ;
    while( !q.empty() ) 
    {
        int now = q.front() ;
        q.pop() ;
        if( now == k )
        {
            cout << step[ now ] ;
            break ;
        }
        if( !vis[ now + 1 ] and now + 1 < 100001 )
        {
            vis[ now + 1 ] = 1 ;
            step[ now + 1 ] = step[ now ] + 1 ;
            q.push( now + 1 ) ;
        }
        if( !vis[ now - 1 ] and now - 1 > 0 )
        {
            vis[ now - 1 ] = 1 ;
            step[ now - 1 ] = step[ now ] + 1 ;
            q.push( now - 1 ) ;
        }
        if( !vis[ now *  2 ] and now * 2 < 100001 )
        {
            vis[ now * 2 ] = 1 ;
            step[ now * 2 ] = step[ now ] + 1 ;
            q.push( now * 2 ) ;
        }
    }
    return 0;
}