1255: Web Navigation

http://tjuacm.chaosheng.top/problem.php?id=1255
一开始没通过是因为访问不到url时就直接把cur设成ignored了，这样会导致“ignored”被当作url压入栈，改了之后就ac了。通过的代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <stack>

using namespace std;

stack<string> back;
stack<string> forw;

int main(){
    string command, url, cur;
    cur = "http://www.acm.org/";
    while(cin >> command){
        if(command == "QUIT") break;
        if(command == "VISIT"){
            cin >> url;
            while(!forw.empty()) forw.pop();
            back.push(cur);
            cur = url;
            cout << cur << endl;
        }
        if(command == "BACK"){
            if(back.empty()){
                cout << "Ignored" << endl;
            }else{
                string tmp = back.top();
                back.pop();
                forw.push(cur);
                cur = tmp;
                cout << cur << endl;
            }    
        }
        if(command == "FORWARD"){
            if(forw.empty()){
                cout << "Ignored" << endl;
            }else{
                string tmp = forw.top();
                forw.pop();
                back.push(cur);
                cur = tmp;
                cout << cur << endl;
            }
        }
    }
    return 0;
}

一开始没通过的：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <stack>

using namespace std;

stack<string> back;
stack<string> forw;

int main(){
    string command, url, cur;
    cur = "http://www.acm.org/";
    while(cin >> command){
        if(command == "QUIT") break;
        if(command == "VISIT"){
            cin >> url;
            while(!forw.empty()) forw.pop();
            back.push(cur);
            cur = url;
        }
        if(command == "BACK"){
            if(back.empty()){
                cur = "Ignored";
            }else{
                string tmp = back.top();
                back.pop();
                forw.push(cur);
                cur = tmp;
            }    
        }
        if(command == "FORWARD"){
            if(forw.empty()){
                cur = "Ignored";
            }else{
                string tmp = forw.top();
                forw.pop();
                back.push(cur);
                cur = tmp;
            }
        }
        cout << cur << endl;
    }
    return 0;
}