LeetCode 278 First Bad Version

LeetCode

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<sup>31</sup> - 1

算法思路：

N/A

注意事项：

1. 题目是先good再bad，所以用first position模板

Python代码：

def firstBadVersion(self, n):
	start, end = 0, n
	while start + 1 < end:
		mid = start + (end - start) // 2
		if isBadVersion(mid):
			end = mid
		else:
			start = mid
	if isBadVersion(start):
		return start
	if isBadVersion(end):
		return end
	return -1

Java代码：

public int firstBadVersion(int n) {
	int start = 1, end = n;
	while(start + 1 < end) {
		int mid = start + (end - start) / 2;
		if(isBadVersion(mid)) 
			end = mid;
		else 
			start = mid;
	}
	if(isBadVersion(start))
		return start;
	else
		return end;
}

算法分析：

时间复杂度为O(logn)，空间复杂度O(1)