Find Smallest Letter Greater Than Target

Problem Statement

You are given an array of characters letters that is sorted in non-decreasing order, and a character target. There are at least two different characters in letters.

Return the smallest character in letters that is lexicographically greater than target. If such a character does not exist, return the first character in letters.

Leetcode Link

Example 1:

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

Explanation: The smallest character that is lexicographically greater than 'a' in letters is 'c'.

Example 2:

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

Explanation: The smallest character that is lexicographically greater than 'c' in letters is 'f'.

Example 3:

Input: letters = ["x","x","y","y"], target = "z"
Output: "x"

Explanation: There are no characters in letters that is lexicographically greater than 'z' so we return letters[0].

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.
• target is a lowercase English letter.

Code

Python

class Solution:
    def nextGreatestLetter(self, letters: List[str], target: str) -> str:
        if target >= letters[-1] or target < letters[0]:
            return letters[0]
        l=0
        h=len(letters)-1
        ans=letters[0]
        while l<=h:
            m=(l+h)>>1
            if letters[m]<=target:
                l=m+1
            else:
                ans=letters[m]
                h=m-1
        return ans