First Bad Version

First Bad Version.png

===================== 解題思路 =====================

二分法檢查 如果 mid 的版本是 bad version 那說明至少 first bad version 會在 mid 或是 mid 之前 所以把範圍縮小到 right = mid, 反之則是 left = mid

===================== C++ code ====================

 /**
 
 
 
  
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.
 */
 
 
 
 
class Solution {
 
 
public:
 
 
/**
 * @param n: An integers.
 * @return: An integer which is the first bad version.
 */
int findFirstBadVersion(int n) {
    // write your code here
    int left = 0, right = n;
    while(left + 1 < right)
    {
        int mid = left + (right - left) / 2;
        if(SVNRepo::isBadVersion(mid)) right = mid;
        else left = mid;
    }
    if(SVNRepo::isBadVersion(left)) return left;
    else return right;
}
 
 
};