744. Find Smallest Letter Greater Than Target

Given a list of sorted characters letters containing only lowercase letters, and given a target letter target, find the smallest element in the list that is larger than the given target.

Letters also wrap around. For example, if the target is target = 'z' and letters = ['a', 'b'], the answer is 'a'.

Examples:

Input:
letters = ["c", "f", "j"]
target = "a"
Output: "c"

Input:ji
letters = ["c", "f", "j"]
target = "c"
Output: "f"

Input:
letters = ["c", "f", "j"]
target = "d"
Output: "f"

Input:
letters = ["c", "f", "j"]
target = "g"
Output: "j"

Input:
letters = ["c", "f", "j"]
target = "j"
Output: "c"

Input:
letters = ["c", "f", "j"]
target = "k"
Output: "c"

Note:

1. letters has a length in range [2, 10000].

2. letters consists of lowercase letters, and contains at least 2 unique letters.

3. target is a lowercase letter.

解法：

用二分查找

class Solution(object):
    def nextGreatestLetter(self, letters, target):
        """
        :type letters: List[str]
        :type target: str
        :rtype: str
        """
        # l = [char for char in letters if char> target]
        # return min(l) if len(l) >0 else letters[0]
        l = 0
        r = len(letters) - 1
        while l <= r:
            m = l + (r - l) / 2
            if letters[m] <= target:
                l = m + 1
            else:
                r = m - 1
        if l < len(letters):
            return letters[l]
        else:
            return letters[0]