3. 抓住那头牛

https://www.acwing.com/problem/content/1102/

#include<iostream>
#include<cstring>
using namespace std;
const int N=200010;

int n,m;
int dist[N];
int q[N];

void bfs(int u)
{
    dist[u]=0;
    int hh=0,tt=0;
    q[0]=u;
    while(hh<=tt)
    {
        int t=q[hh++];
        if(t>2*max(n,m)) continue;
        if(dist[t+1]==-1) 
        {
            dist[t+1]=dist[t]+1;
            q[++tt]=t+1;
        }
        if(dist[t-1]==-1)
        {
            dist[t-1]=dist[t]+1;
            q[++tt]=t-1;
        }
        if(dist[2*t]==-1)
        {
            dist[2*t]=dist[t]+1;
            q[++tt]=2*t;
        }
    }
}

int main()
{
    cin>>n>>m;
    memset(dist,-1,sizeof dist);
    bfs(n);
    cout<<dist[m]<<endl;
    return 0;
}