LeetCode: 278. First Bad Version

题目描述

You are a product manager and currently leading a team to develop a new product. Unfortunately, the latest version of your product fails the quality check. Since each version is developed based on the previous version, all the versions after a bad version are also bad.

Suppose you have n versions [1, 2, ..., n] and you want to find out the first bad one, which causes all the following ones to be bad.

You are given an API bool isBadVersion(version) which will return whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

Example:

Given n = 5, and version = 4 is the first bad version.

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

Then 4 is the first bad version.

解题思路 —— 二分查找

  • isBadVersion(mid) == true, 则最小的坏版本在 [beg, mid] 区间
  • 否则，最小的坏版本在 (mid, end) 区间

AC 代码

// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution
{
private:
    int firstBadVersion(long long int beg, long long int end)
    {
        if(beg >= end) return -1;
      
        long long int mid = (beg+end)/2;
        if(isBadVersion(mid) == true)
        {
            int ans = firstBadVersion(beg, mid);
            if(ans == -1)
            {
                return mid;
            }
            else
            {
                return ans;
            }
        }

        return firstBadVersion(mid+1, end);
    }
public:
    int firstBadVersion(int n) 
    {
        return firstBadVersion(0, (long long int)n+1);
    }
};