LintCode-First Bad Version

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

You can determine whether a version is bad by the following interface: 

 

Java:
    public VersionControl {
        boolean isBadVersion(int version);
    }
C++:
    class VersionControl {
    public:
        bool isBadVersion(int version);
    };
Python:
    class VersionControl:
        def isBadVersion(version)

 

Find the first bad version.

Note

You should call isBadVersion as few as possible. 

Please read the annotation in code area to get the correct way to call isBadVersion in different language. For example, Java is VersionControl.isBadVersion.

Example

Given n=5

Call isBadVersion(3), get false

Call isBadVersion(5), get true

Call isBadVersion(4), get true

return 4 is the first bad version

Challenge

Do not call isBadVersion exceed O(logn) times.

Solution:

/**
 * public class VersionControl {
 *     public static boolean isBadVersion(int k);
 * }
 * you can use VersionControl.isBadVersion(k) to judge wether
 * 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) {
        if (n==1) return 1;

        int start = 1;
        int end = n;
        while (start<=end){
            int mid = start + (end-start)/2;

            if (!VersionControl.isBadVersion(mid))
                start = mid +1;
            else {
                if (mid==1 || !VersionControl.isBadVersion(mid-1))
                    return mid;
                else end = mid-1;
            }
        }

        return -1;
    }
}