poj 1028

　　他就是个栈，栈就随便写了，贴代码

#include <iostream>
#include <stack>
#include <string>
using namespace std;
int main(){
    stack<string> stack1,stack2;            string str,s1,s2;  
    str="http://www.acm.org/";              stack1.push(str);
    while(cin>>s1){
        if(s1=="QUIT")                      break;
        if(s1=="VISIT"){
            cin>>s2;   cout<<s2<<endl;        stack1.push(s2);  
            while(!stack2.empty())          stack2.pop();
        } else if(s1=="BACK"){
            if(stack1.size()>=2){  
            stack2.push(stack1.top());      stack1.pop();
            cout<<stack1.top()<<endl;    
            } else                          puts("Ignored");
          } else {
            if(!stack2.empty()){
                cout<<stack2.top()<<endl;
                stack1.push(stack2.top());  stack2.pop();      
            } else                           puts("Ignored");
        }
    }    
    return 0;
}