I've got TLE on test no. 28; here is the "main" piece of code, im wondering why it is slow. Isn't O(n) complexity? Thanks to everyone who help me in advance. Here is the code

void solve()
{
    int p=1;
    f(i,1,n)
    {
        f(j,p,b[i])
        {
            if (!used[j]) { s.push(j); used[j]=1; }
        }
        if (s.top()==b[i]) s.pop();
        else
        {
            cout<<"Cheater\n";
            return;
        }

        p=b[i];
    }

    if (s.empty()) cout<<"Not a proof\n";
    else cout<<"Cheater\n";
}

Here f(i,beg,end) == for(int i=beg; i<=end; i++), s-STL stack<int>, b[] contains the input and used[] is a control array which shows me if ball with number i was already used. Can someone give me a test on which this code will TL ?

To anyone who has the same problem, let's try this test:
100000
1
100000
2
99999
3
99998
...
49999
50000

It gives me TL cause of the inside cycle f(i,p,b[i])

I tried this test, but there is TL.
In VS this test takes less then 40 ms.
What's this test 28?