vector<int>v;
    printf("%d\n",v.size());
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    v.pop_back();
    
    printf("%d\n",v.size());

犹点离谱。给了个负数，还不会报错RE。啥原因？