Leetcode #744: Find Smallest Letter Greater Than Target
In this guide, we solve Leetcode #744 Find Smallest Letter Greater Than Target in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
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.
Quick Facts
- Difficulty: Easy
- Premium: No
- Tags: Array, Binary Search
Intuition
The problem structure suggests a monotonic decision, which makes binary search a natural fit.
By halving the search space each step, we reach the answer efficiently.
Approach
Search either directly on a sorted array or on the answer space using a check function.
Each check is fast, and the logarithmic search keeps the overall runtime low.
Steps:
- Define the search bounds.
- Check the mid point condition.
- Narrow the bounds until convergence.
Example
Input: letters = ["c","f","j"], target = "a"
Output: "c"
Explanation: The smallest character that is lexicographically greater than 'a' in letters is 'c'.
Python Solution
class Solution:
def nextGreatestLetter(self, letters: List[str], target: str) -> str:
i = bisect_right(letters, ord(target), key=lambda c: ord(c))
return letters[i % len(letters)]
Complexity
The time complexity is , where is the length of letters. The space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.