LeetCode 278. First Bad Version

原题链接在这里：https://leetcode.com/problems/first-bad-version/

题目：

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 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.

题解：

Binary Search. 查mid是不是，若是bad version, 就在左边继续查，若不是bad version, 就在右边继续查找。

最后出了loop是返回left还是right最好举几个例子.

Note: 求中点mid时, 用 mid = left +(right - left)/2, 不能使用 mid = (left + right)/2 因为loop 过程中(left + right)可能超过 Integer.MAX_VALUE, overflow了。

Time Complexity: O(logn). Space: O(1).

AC Java:

/* 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 = 1;
        int right = n;
        int res = 0;
        while(left<=right){
            int mid = left + (right - left)/2;
            if(isBadVersion(mid)){
                right = mid-1;
            }else{
                left = mid+1;
            }
        }
        return left;
    }
}

 类似Search Insert Position, Find First and Last Position of Element in Sorted Array, Guess Number Higher or Lower.