栈练习2

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<stack>
 4 using namespace std;
 5 int main()
 6 {
 7     long long n,k,a,t=1;
 8     stack<int>ss;
 9     cin>>n;
10     while(n--)
11     {
12         cin>>k;
13         if(k==1)
14         {
15             cin>>a;
16             ss.push(a);
17             continue;
18         }
19         if(k==2&&ss.empty())
20         {
21             t=0;break;//空的时候还出栈就不可以了...
22         }
23         ss.pop();
24     }
25     if(ss.empty()||t==0)  cout<<"impossible!"<<endl;
26     else cout<<ss.top();
27     return 0;
28  }