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 1 #include <iostream>
 2 #include <vector>
 3 using namespace std;
 4 vector<string> vs;
 5 int main(){
 6     string cmd;
 7     string addr;
 8     int index = 0;
 9     vs.push_back(string("http://www.acm.org/"));
10     while(cin>>cmd && cmd != "QUIT"){
11         if(cmd == "VISIT"){
12             cin>>addr;
13             /* 这样会wa,因为pop_back()操作会影响到for循环中的条件vs.size()的改变.
14             if(index != vs.size()-1){
15                 for(int i=0;i<vs.size()-index-1;++i){
16                     vs.pop_back();
17                 }
18             }
19             */
20             while(index < vs.size()-1){
21                 vs.pop_back();
22             }
23             vs.push_back(addr);
24             index++;
25             cout<<vs[index]<<endl;
26         }else if(cmd == "BACK"){
27             if(index==0){
28                 cout<<"Ignored"<<endl;
29             }else{
30                 index--;
31                 cout<<vs[index]<<endl;
32             }
33         }else if(cmd == "FORWARD"){
34             if(index==vs.size()-1){
35                 cout<<"Ignored"<<endl;
36             }else{
37                 index++;
38                 cout<<vs[index]<<endl;
39             }
40         }else{
41             cout<<"Ignored"<<endl;
42         }
43     }
44     return 0;
45 }