[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.

解题思路

直接线性查找则调用API次数过多，因此采用二分查找。
此处需注意的一个问题是，求mid时宜选择mid = left + (right - left) / 2，以避免溢出问题。

实现代码

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;
    }
}