class Solution {
public:
    char bsf(vector<char> letters, char target)
    {
        char ans=letters[0];
        int n=letters.size();
        int l=0,h=n-1;
        int mid=l+(h-l)/2;
        while(l<=h)
        {
            if(letters[mid]-'a'>target-'a') 
            {
                ans=letters[mid];
                h=mid-1;
            }
            else l=mid+1;
            mid=l+(h-l)/2;
        }
        return ans;
    }
    
    char nextGreatestLetter(vector<char>& letters, char target) {
        char result=bsf(letters, target);
        return result;
    }
};