Leetcode #2255: Count Prefixes of a Given String
In this guide, we solve Leetcode #2255 Count Prefixes of a Given String 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 a string array words and a string s, where words[i] and s comprise only of lowercase English letters. Return the number of strings in words that are a prefix of s.
Quick Facts
- Difficulty: Easy
- Premium: No
- Tags: Array, String
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: words = ["a","b","c","ab","bc","abc"], s = "abc"
Output: 3
Explanation:
The strings in words which are a prefix of s = "abc" are:
"a", "ab", and "abc".
Thus the number of strings in words which are a prefix of s is 3.
Python Solution
class Solution:
def countPrefixes(self, words: List[str], s: str) -> int:
return sum(s.startswith(w) for w in words)
Complexity
The time complexity is , where and are the lengths of the array words and the string s, respectively. 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.