class Solution:
    '''
    Time Complexity: O(log n)
    Space Complexity: O(1)
    where, n is the length of letters
    Solved in 13 min 24 secs, all by yourself! Good Job!
    '''
    def nextGreatestLetter(self, letters: List[str], target: str) -> str:
        l, r = 0, len(letters)-1
        while l < r:
            m = (l+r)//2
            if letters[m] <= target:
                l = m+1
            else:
                r = m
        if letters[l] <= target and l == len(letters)-1:
            return letters[0]
        return letters[l]

from bisect import bisect
class Solution:
    '''
    Time Complexity: O(log n)
    Space Complexity: O(1)
    where, n is the length of letters
    Cleaner code!
    '''
    def nextGreatestLetter(self, letters: List[str], target: str) -> str:
        i = bisect(letters, target)
        if i == len(letters):
            return letters[0]
        return letters[i]