1021. Couples

#include <iostream>
#include <stack>
using namespace std;
int main(){
    
    int husband;
    int wife;
    int a[200001];
    int n;
    
    while(1){           
    cin >> n;
    if (n==0)
       break;
    
    for(int i=1;i<=n;i++){              
       cin >> husband >> wife;
       a[husband]=i;
       a[wife]=i;
    }
    
    stack <int> couples;
    couples.push(a[1]);
    //1.重点：采用STL的stack，需要包含头文件 #include <stack>
    //2.重点：记住stack的基本函数 pop()、push()、empty()、top(} 
    for(int j=2;j<=2*n;j++){
       if(!couples.empty()&& a[j]==couples.top())
          couples.pop();
       else
          couples.push(a[j]);                   
    }
    
    if(!couples.empty())
      cout << "No" << endl;
    else
      cout << "Yes" << endl;
      
   } 

    return 0;
}