Brooke

一、題目

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 <= 231 - 1

 

二、程式作法

/* The isBadVersion API is defined in the parent class VersionControl.
      bool IsBadVersion(int version); */
/*寫法一*/
public class Solution : VersionControl
{
    public int FirstBadVersion(int n)
    {
        if (IsBadVersion(1))
            return 1;

        int left = 1;
        int right = n;
        int pivot = 1;

        while (left <= right)
        {
            pivot = left + (right - left + 1) / 2;
            if (IsBadVersion(pivot - 1) == false && IsBadVersion(pivot) == true)
                return pivot;

            if (IsBadVersion(pivot) == true)
                right = pivot;
            else
                left = pivot;
        }
        return 0;
    }
}

/*寫法二*/
/*
public class Solution : VersionControl
{
    public int FirstBadVersion(int n)
    {
        int pivot = 1;
        int lowBound = 1;
        int upBound = n;

        while (lowBound <= upBound)
        {
            pivot = lowBound + (upBound - lowBound) / 2;

            if (IsBadVersion(pivot) == true && IsBadVersion(pivot - 1) == false) return pivot;

            if (upBound == 1) return 1;

            if (IsBadVersion(pivot))
                upBound = pivot;
            else
                lowBound = pivot + 1;
        }
        return pivot;
    }
}
*/

 

三、思路筆記

1、一開始還看不懂題目規範，原來是輸入一版本數，找出第一版壞掉的版本。

2、那就使用 binary search 來處理。

我們要找到第一版壞掉的版本時，則要先確認其上一版是好的。