lintcode:First Bad Version

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.

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.

Note
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)

Challenge
You should call isBadVersion as few as possible.

/**
 * class SVNRepo {
 *     public:
 *     static bool isBadVersion(int k);
 * }
 * you can use SVNRepo::isBadVersion(k) to judge whether 
 * the kth code version is bad or not.
*/
class Solution {
public:
    /**
     * @param n: An integers.
     * @return: An integer which is the first bad version.
     */
    int findFirstBadVersion(int n) {
        // write your code here
        int l=1,r=n;
        while(l<=r){
            int mid=l+(r-l)/2;
            if(SVNRepo::isBadVersion(mid)){
                r=mid-1;
            }else{
                l=mid+1;
            }
        }
        return l;
    }
};

为什么最后返回l，而不是r？

因为，循环里的最后一步l,r都停留在出错的索引那个位置，接着mid==l==r，则if判断为正，r=mid-1；不符合循环条件，循环停止。
也就是说l指示的是正确的出错位置。OK