The celebrity problem

/*

Logic is :there can be only on celebrity possible,because for being another celebrity,first celebrity must know the other celebrity
and if first celebrity knows the second celebrity,then first celebrity can't be celebrity because he knows someone else.
so using stack,i found that one probable celebrity.
*/
class Solution
{
    public:
    int celebrity(vector<vector<int> >& arr, int n) {
        int i=0;
        stack <int> s;
        while(i<n){
            s.push(i);
            i++;
        }
        int a,b;
        while(s.size()>=2){  //either he knows or get known,if knows then it can be celebrity else possibility of being celeb
            a=s.top();s.pop();
            b=s.top();s.pop();
            if(arr[a][b]==1){  //a knows b so a can't be celebrity so b chances of being celebrity
                s.push(b);
                continue;
            }
            else{              //a doesn't knows b so a can be celebrity.b can't because a doesn't know b.
                s.push(a);
            }
        }
        a=s.top();
        for(i=0;i<n;i++){  //check fro probable celebrity that he doesnt; know anyone and known by everyone
            if(i==a)
               continue;
            if(arr[a][i]==1)
                return -1;
            if(arr[i][a]==0)
                return -1;
        }
        return a;
    }
};