744. 寻找比目标字母大的最小字母

class Solution:
    def nextGreatestLetter(self, letters: List[str], target: str) -> str:
        # 类似于找插入位置
        left = 0 
        right = len(letters)-1
        while left<=right:
            half = left + (right-left)//2
            if letters[half] > target:
                right = half - 1
            else:
                left = half + 1
        return letters[left] if left !=len(letters) else letters[0]