First Bad Version

• LintCode: No.74-First Bad Version

• Problem:

  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.
  

• Analysis:

    Using Binary search to find the first bad version.  
    Case 1. mid is TRUE  
        find left side  
    Case 2. mid is FALSE  
        find right side
  

• Java:

    /\*\*
  

  • 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 if(n <= 0) { return 0;
      } return helper(0, n); } private int helper(int left, int right) { if (left + 1 == right) {

       return SVNRepo.isBadVersion(right) ? right : left;
      

      } int mid = (left + right) / 2; if (SVNRepo.isBadVersion(mid)) {

       return helper(left, mid);
      

      } else {

       return helper(mid, right);
      

      } } }

• Time Complexity:

  • O(logn)