Leetcode #1392: Longest Happy Prefix
In this guide, we solve Leetcode #1392 Longest Happy Prefix 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
A string is called a happy prefix if is a non-empty prefix which is also a suffix (excluding itself). Given a string s, return the longest happy prefix of s.
Quick Facts
- Difficulty: Hard
- Premium: No
- Tags: String, String Matching, Hash Function, Rolling Hash
Intuition
We need to scan characters while tracking positions or counts.
A simple state machine keeps the logic precise.
Approach
Iterate through the string once and update the state for each character.
Use a map or array if you need fast lookups.
Steps:
- Iterate through characters.
- Maintain necessary state.
- Build or validate the output.
Example
Input: s = "level"
Output: "l"
Explanation: s contains 4 prefix excluding itself ("l", "le", "lev", "leve"), and suffix ("l", "el", "vel", "evel"). The largest prefix which is also suffix is given by "l".
Python Solution
class Solution:
def longestPrefix(self, s: str) -> str:
for i in range(1, len(s)):
if s[:-i] == s[i:]:
return s[i:]
return ''
Complexity
The time complexity is , and the space complexity is . 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.