队列的元素反复入队有问题，另外a-1>=0，修改后代码

#include<bits/stdc++.h>
using namespace std;

void bfs(int s, int t, int& ans) {
    vector<int> visit(100001, 0);
    if(s == t) return;
    queue<int> q;
    q.push(s); 
    visit[s] = 0;
    while(!q.empty()) {
        int a = q.front();
        q.pop();
        if(a == t) {
			ans = visit[a];
            return;
        } else {
            if(a + 1 < 100001 && visit[a + 1] == 0) {
                q.push(a + 1);
                visit[a + 1] = visit[a]+1;
            } 
            if(a - 1 >= 0 && visit[a - 1] == 0) {
                q.push(a - 1);
                visit[a - 1] = visit[a]+1;
            }
            if(a * 2 < 100001 && visit[a * 2] == 0) {
                q.push(a * 2);
                visit[a * 2] = visit[a]+1;
            }
        }
    }
}

int main() {
    int s, t;
    while(cin>>s>>t){
        int ans = 0;
        bfs(s, t, ans);
        cout<<ans<<endl;
    }
    return 0;
}