First Bad Version

這道題用Binar Search到是沒有什麼難度，主要的就是判斷中間點的時候容易出錯。起初我用的是left = 1, high = n; mid = (left + high)/2,但是不AC。後來發現當起始下標爲1的時候，這種求mid會Overflow,這種情況是適用於left從0開始的。從1開始的話需要使用mid = left + (high -left) / 2來計算。即可AC：

bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int low = 1;
        int high = n;
        int mid;
        while(low < high)
        {
            mid  = low + (high-low)/2;   //(left+high)/2 will result a overflow
            if(isBadVersion(mid))        // is bad
            {
                if(mid <= 1)
                    return mid;
                else
                {
                    if(!isBadVersion(mid-1)) //前面一個不壞
                        return mid;
                    else
                        high = mid;
                }
            }
            else
                low = mid + 1;
        }
        return high;
    }
};