278. First Bad Version

二分法:

// Forward declaration of isBadVersion API.
bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n)
    {
        int left=1,right=n;
       
        while(left<=right)
        {
            int mid=left+(right-left)/2;  //以后用此法求mid，避免溢出
            if(isBadVersion(mid))
            {
                right=mid-1;
            }
            else
            {
                left=mid+1;
            }
          
        }
        return left;
    }
};