1253：抓住那头牛

题目分析：

转移三种状态：-1，+1，*2
标记已经走过的点，并记录步数

代码如下:

#include<iostream>
#include<cstring>
#include<cstdio>
#include<queue>

using namespace std;

#define x first
#define y second
#define el endl
#define pii pair<int,int>
#define mm(a,x) memset(a,x,sizeof a)
#define mk make_pair

const int N = 1e5 + 10;

int n,k;
int dist[N];

int bfs(int sx){
    mm(dist,-1);
    queue<int> q;
    q.push(sx);
    dist[sx] = 0;
    while(q.size()){
        int t = q.front();
        q.pop();

        if(t == k) return dist[k];

        int dx[3] = {t - 1,t + 1,t * 2};
        for(int i = 0; i < 3; i ++ ){
            int a = dx[i];
            if(a < 0 || a > 1e5) continue;
            if(dist[a] != -1) continue;
            dist[a] = dist[t] + 1;
            q.push(a);
        }

    }
}

int main(){
    cin >> n >> k;
    cout<<bfs(n);    
    return 0;
}