[LeetCode] 278. First Bad Version

轉自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 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.

<Solution>

這題用二分搜尋法解即可

思路如下

• 如果 mid 是 false，代表第一個 bad version 會在前半部

• 反之，第一個 bad version 會在後半部

code 如下

C++

	// Forward declaration of isBadVersion API.
	bool isBadVersion(int version);
	class Solution {
	public:
	int firstBadVersion(int n) {
	int start = 1, end = n;
	int mid;
	while(start < end) {
	mid = start + (end - start) / 2;
	if(isBadVersion(mid)) {
	end = mid;
	}
	else {
	start = mid+1;
	}
	}
	return start;
	}
	};

view raw firstBadVersion.cpp hosted with ❤ by GitHub

kotlin

	/* The isBadVersion API is defined in the parent class VersionControl.
	def isBadVersion(version: Int): Boolean = {} */
	class Solution: VersionControl() {
	override fun firstBadVersion(n: Int) : Int {
	var left = 1
	var right = n
	while(left < right) {
	val mid = left + (right - left) / 2
	if(isBadVersion(mid)) {
	right = mid
	} else {
	left = mid+1
	}
	}
	return left
	}
	}

view raw first_bad_version.kt hosted with ❤ by GitHub