LintCode: First Bad Version

The code base version is an integer start from 1 to n. One day, someone committed a bad version in the code case, so it caused this version and the following versions are all failed in the unit tests. Find the first bad version.

You can call isBadVersion to help you determine which version is the first bad one. The details interface can be found in the code's annotation part.

/**
 * 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) {
        // write your code here
        int start = 1, end = n;
        while (start + 1 < end) {
            int mid = start + (end -start) / 2;
            if (SVNRepo.isBadVersion(mid)) {
                end = mid;
            } else {
                start = mid;
            }
        }
        
        if (SVNRepo.isBadVersion(start)) {
            return start;
        }
        return end;
    }
}

Use the binary search to solve this problem, the basic idea is to find the first version that is the bad version.