LeetCode 278. 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.

分析：本题大意就是找到第一个使 isBadVersion(version) 返回为false的索引值。本题与(M) Search Insert Position 类似

题目要求减少调用API的次数。可以选择二叉查找法。

代码：

/* 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 l = 1, r = n;
        int m;        
        while(l < r){
            m = l + (r - l) / 2;// 二叉查找
            boolean f1 = isBadVersion(m);
            if(f1) r = m;// 若当前 m 对应为假，则记录下来
            else l = m + 1;// 否则循环判断下半区
        }// 循环结束，r 一定指向第一个badVersion
        return r;
    }
}