[leetcode] 278. First Bad Version

题目

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 <= 2^31 - 1

思路

二分搜索，如果是坏版本，则移动右指针至中间位置，否则移动左指针至中间的后一位，如此反复，直至左右指针相遇，此时左指针的位置即是第一个坏版本。

代码

python版本：

# The isBadVersion API is already defined for you.
# def isBadVersion(version: int) -> bool:

class Solution:
    def firstBadVersion(self, n: int) -> int:
        l,r=0,n+1
        while l<r:
            mid=(l+r)//2
            if isBadVersion(mid):
                r=mid
            else:
                l=mid+1
        return l