此時我們可以斷定4是第一個錯誤的版本號

/**
 * public class SVNRepo {
 *     public static boolean isBadVersion(int k);
 * }
 * you can use SVNRepo.isBadVersion(k) to judge whether 
 * the kth code version is bad or not.
*/
class Solution {
    /**
     * @param n: An integers.
     * @return: An integer which is the first bad version.
     */
    public int findFirstBadVersion(int n) {
          int min = 1, max = n, mid = 0;
        while(min <= max){
            mid = min + (max - min) / 2;
            if(SVNRepo.isBadVersion(mid)){
                max = mid - 1;
            } else {
                min = mid + 1;
            }
        }
        return min;
    }
}