278. First Bad Version

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 returns whether version is bad. Implement a function to find the first bad version. You should minimize the number of calls to the API.

 

Example 1:

Input: n = 5, bad = 4
Output: 4
Explanation:
call isBadVersion(3) -> false
call isBadVersion(5) -> true
call isBadVersion(4) -> true
Then 4 is the first bad version.

Example 2:

Input: n = 1, bad = 1
Output: 1

 

Constraints:

• 1 <= bad <= n <= 231 - 1

 

---

#https://leetcode.com/problems/first-bad-version/
'''
1. 아이디어 :
    start, end로 두 포인터를 설정하고, isBadVersion(mid)를 통해 mid가 bad인지 아닌지 확인한다.
    확인 후, False면 start 포인터를, True면 end 포인터를 mid+-1로 옮긴다.
2. 시간복잡도 :
    1) O(logn)
3. 자료구조 :
    1) 배열
'''

# The isBadVersion API is already defined for you.
# def isBadVersion(version: int) -> bool:
class Solution:
    def firstBadVersion(self, n: int) -> int:
        start=0
        end=n
        mid=0
        while start<=end:
            mid=(start+end)//2
            check=isBadVersion(mid)
            if check:
                end=mid-1
            else:
                start=mid+1
        print(start,mid,end)
        return start