find celebrity

```swift
knows(i, j)
i 知道 j
        for i in 1..< n {
            if knows(celeb, i) {
                celeb = i
            }
        }
        for j in 0..< n where celeb != j {
            if (knows(celeb, j)) || !knows(j, celeb) {// there is no celeb
                return -1
            }
        }
        return celeb