278. First Bad Version

Easy
​
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
​

解題

使用了類似Binary search的技巧。直接遞迴0~n的話會出現Time limit exceed的錯誤。
Runtime: 0 ms, faster than 100.00%
Memory Usage: 1.9 MB, less than 87.99%
/**
* Forward declaration of isBadVersion API.
* @param version your guess about first bad version
* @return true if current version is bad
* false if current version is good
* func isBadVersion(version int) bool;
*/
​
func firstBadVersion(n int) int {
start := 0
end := n
for start <= end {
mid := (start + end) / 2
if isBadVersion(mid) {
if !isBadVersion(mid - 1) {
return mid
} else {
end = mid-1
}
} else {
start = mid+1
}
}
return 0
}
​
​