题目描述：

LeetCode 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:
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.

Python代码：

``````class Solution(object):
def nextGreatestLetter(self, letters, target):
"""
:type letters: List[str]
:type target: str
:rtype: str
"""
letters = set(map(ord, letters))
for x in range(1, 27):
candidate = ord(target) + x
if candidate > ord('z'):
candidate -= 26
if candidate in letters:
return chr(candidate)
``````

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