If a knows b, then a will not the celebrity, if a does not know b, then b will not be the celebrity. So we could find one possible person from the n persons.

Next step is that we check this person does not know anyone else and all of the other n-1 ppl know this person.

```
bool knows(int a, int b);
class Solution {
public:
int findCelebrity(int n) {
int t = 0;
for(int i = 1; i < n; ++i){
if(knows(t,i)){t = i;}
}
for(int i = 0; i < n; ++i){
if(i != t){
if(!knows(i,t) || knows(t,i)) return -1;
}
}
return t;
}
};
```