First Bad Version

/* The isBadVersion API is defined in the parent class VersionControl.
      boolean isBadVersion(int version); */

public class Solution extends VersionControl {
    public int firstBadVersion(int n) {
        return findFirst(1, n);
    }
    public int findFirst(int left, int right){
        if(left >= right){
            return left;
        }
        int mid = left + (right - left) / 2;
        if(isBadVersion(mid)){
            return findFirst(left, mid);
        }
        else{
            return findFirst(mid+1, right);
        }
    }
}