First Bad Version

Description

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.

Link

Lintcode_ladder

Method

1. Binary search
2. x

Example

1. 1

/**
 * class SVNRepo {
 *     public:
 *     static bool isBadVersion(int k);
 * }
 * you can use SVNRepo::isBadVersion(k) to judge whether 
 * the kth code version is bad or not.
*/
//using namespace::SVNRepo;
class Solution {
public:
    /**
     * @param n: An integers.
     * @return: An integer which is the first bad version.
     */
    int findFirstBadVersion(int n) {
        // write your code here
        if (n == 0) {
           return -1;
        }
        int start = 1;
        int end = n;
        while (start+1 < end) {
            int mid = start + (end-start)/2;
            if (SVNRepo::isBadVersion(mid)) {
                end = mid;
            } else {
                start = mid;
            }
        }
        // if the bad num must be here, we can just check once
        if (SVNRepo::isBadVersion(start)) {
            return start;
        } 
        else {
            return end;
        }
        // if (SVNRepo::isBadVersion(end)) {
        //     return end;
        // }
        return -1;
    }
};

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