do the first loop with increasing order and find candidate1， do the second loop with reverse order and find candidate2， if candidate1 == candidate2，candidate1 is the celebrity ，otherwise return -1

```
bool knows(int a, int b);
class Solution {
public:
int findCelebrity(int n) {
int candidate1 = 0;
for (int i = 1; i < n; ++i) {
if (knows(candidate1, i)) {
candidate1 = i;
}
}
int candidate2 = n-1;
for (int i = n-2; i >=0; --i) {
if (knows(candidate2, i)) {
candidate2 = i;
}
}
return candidate1 == candidate2? candidate1 : -1;
}
};
```