POJ1363-Rails

http://poj.org/problem?id=1363

#include<cstdio>
#include<stack>
using namespace std;
int a[1000];
int main(void)
{
    int n;
    while(scanf("%d",&n),n)
    {
        int x,t;
        stack<int> q;
        while(scanf("%d",&x)&&x)
        {
            a[1]=x;
            for(int i=2;i<=n;i++)  
               scanf("%d",&a[i]);
            int num=1;
            for(int i=1;i<=n;i++)
            {
                q.push(i);
                t=q.top();
                while(t==a[num]&&num<=n)
                {
                    num++;
                    q.pop();
                    if(!q.empty()) 
                       t=q.top();
                }
            }
            if(num>n)   
               printf("Yes\n");
            else 
               printf("No\n");
        }
        printf("\n");
    }
    return 0;
}