First Bad Version——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.

题目大意：n个版本，如果某个版本是坏的，后面都是坏的，找出第一个坏的。

解题思路：二分查找。

public class Solution extends VersionControl {
    public int firstBadVersion(int n) {
        if (isBadVersion(1)){
            return 1;
        }
        int lo = 0;
        int hi = n - 1;
        while (lo <= hi) {
            int mid = (lo + hi) >>> 1;
            if (!isBadVersion(mid + 1) && isBadVersion(mid + 2)) {
                return mid + 2;
            }
            if (mid > 0 && !isBadVersion(mid) && isBadVersion(mid + 1)) {
                return mid + 1;
            } else if (isBadVersion(mid + 1)) {
                hi = mid - 1;
            } else if (!isBadVersion(mid + 1)) {
                lo = mid + 1;
            }
        }
        return -1;
    }
}