POJ1028 Web Navigation【栈模拟：按照题目的做就行】

#include<iostream>
#include<stack>
#include<string>
#pragma warning (disable:4996)

using namespace std;

#define debug(x) cout<<#x<<": "<<(x)<<endl;

int main() {

  //freopen("../in1.txt","r",stdin);

  string m;

  stack<string> bst;
  stack<string> fst;

  string cur = "http://www.acm.org/";

  while (cin>>m)  {
    
    if (m == "QUIT") {
      break;
    }

    if (m == "VISIT") {
      string u;
      cin >> u;

      bst.push(cur);
      cur = u;
      cout << cur << endl;
      
      while (!fst.empty())  {
        fst.pop();
      }
    }
    else if(m=="BACK"){
      
      if (bst.empty()) {
        cout << "Ignored" << endl;
      }
      else {
        fst.push(cur);
        cur = bst.top();
        bst.pop();
        cout << cur << endl;
      }
    }
    else if(m=="FORWARD"){
      
      if (fst.empty()) {
        cout << "Ignored" << endl;
      }
      else {
        bst.push(cur);
        cur = fst.top();
        fst.pop();
        cout << cur << endl;
      }
    }
  }
  return 0;
}