74. First Bad Version（二分法）

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本文介绍了一个经典的二分查找算法的应用实例：通过调用接口确定软件仓库中首次出现故障的版本。利用二分查找方法，可以在最少的接口调用次数内找到故障版本。

Description

The code base version is an integer start from 1 to n. One day, someone committed a bad version in the code case, so it caused this version and the following versions are all failed in the unit tests. Find the first bad version.

You can call isBadVersion to help you determine which version is the first bad one. The details interface can be found in the code's annotation part.

Notice

Please read the annotation in code area to get the correct way to call isBadVersion in different language. For example, Java is SVNRepo.isBadVersion(v)

Example

Given n = 5:

isBadVersion(3) -> false
isBadVersion(5) -> true
isBadVersion(4) -> true

Here we are 100% sure that the 4th version is the first bad version.

Challenge

You should call isBadVersion as few as possible.

Solution

这道题是经典二分法例题的变形，只不过将查找元素变为了调用接口。

Java

/**
 * public class SVNRepo {
 *     public static boolean isBadVersion(int k);
 * }
 * you can use SVNRepo.isBadVersion(k) to judge whether 
 * the kth code version is bad or not.
*/

public class Solution {
    /*
     * @param n: An integer
     * @return: An integer which is the first bad version.
     */
    public int findFirstBadVersion(int n) {
        // write your code here
        int start = 1, end = n;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (SVNRepo.isBadVersion(mid)) {
                end = mid;
            } else {
                start = mid;
            }
        }
        if (SVNRepo.isBadVersion(start)) {
            return start;
        }
        //若不是start位置则一定在end位置，由题意知不存在返回-1的情况
        return end;
    }
}