[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.

 Notice

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

Example

Given n = 5:

isBadVersion(3) -> false
isBadVersion(5) -> true
isBadVersion(4) -> true

Here we are 100% sure that the 4th version is the first bad version.

Challenge 

You should call isBadVersion as few as possible.

This problem can be converted to given a sorted sequence, find the first element that meets XXXX requirement.

Binary search gives us O(logn) runtime.

public int findFirstBadVersion(int n) {
    int start = 1, end = n;
    while(start < end - 1) {
        int mid = start + (end - start) / 2;
        if(SVNRepo.isBadVersion(mid)) {
            end = mid;
        }
        else {
            start = mid + 1;
        }
    }
    if(SVNRepo.isBadVersion(start)) {
        return start;
    }
    return end;
}

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