Problem Statement

Given an array of lowercase letters sorted in ascending order, find the smallest letter in the given array greater than a given key.

Assume the given array is a circular list, which means that the last letter is assumed to be connected with the first letter. Put simply: if the key is greater than or equal to the last letter of the array, the search wraps around, and the answer is the first letter of the array.

Write a function to return the next letter of the given key.

Example 1:

Input: ['a', 'c', 'f', 'h'], key = 'f'
Output: 'h'
Explanation: The smallest letter greater than 'f' is 'h' in the given array.

Example 2:

Input: ['a', 'c', 'f', 'h'], key = 'b'
Output: 'c'
Explanation: The smallest letter greater than 'b' is 'c'.

Example 3:

Input: ['a', 'c', 'f', 'h'], key = 'm'
Output: 'a'
Explanation: As the array is assumed to be circular, the smallest letter greater than 'm' is 'a'.

Example 4:

Input: ['a', 'c', 'f', 'h'], key = 'h'
Output: 'a'
Explanation: As the array is assumed to be circular, the smallest letter greater than 'h' is 'a'.

Constraints:

• 2 <= letters.length <= 10⁴
• letters[i] is a lowercase English letter.
• letters is sorted in non-decreasing order.
• letters contains at least two different characters.
• key is a lowercase English letter.

Try it yourself

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