[LeetCode] First Bad Version

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.

解題思路

直接線性查找則調用API次數過多，因此采用二分查找。
此處需注意的一個問題是，求mid時宜選擇mid = left + (right - left) / 2，以避免溢出問題。

實現代碼

C++：

// Runtime: 0 ms
// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int left = 0;
        int right = n;
        while (left <= right) 
        {
            int mid = left + (right - left) / 2;
            if (isBadVersion(mid))
            {
                right = mid - 1;
            }
            else
            {
                left = mid + 1;
            }
        }

        return left;
    }
};

Java:

// Runtime: 19 ms
/* The isBadVersion API is defined in the parent class VersionControl.
      boolean isBadVersion(int version); */

public class Solution extends VersionControl {
    public int firstBadVersion(int n) {
        int left = 0;
        int right = n;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            if (isBadVersion(mid)) {
                right = mid - 1;
            }
            else {
                left = mid + 1;
            }
        }

        return left;
    }
}