Leetcode #2047: Number of Valid Words in a Sentence
In this guide, we solve Leetcode #2047 Number of Valid Words in a Sentence 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 sentence consists of lowercase letters ('a' to 'z'), digits ('0' to '9'), hyphens ('-'), punctuation marks ('!', '.', and ','), and spaces (' ') only. Each sentence can be broken down into one or more tokens separated by one or more spaces ' '.
Quick Facts
- Difficulty: Easy
- Premium: No
- Tags: 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: sentence = "cat and dog"
Output: 3
Explanation: The valid words in the sentence are "cat", "and", and "dog".
Python Solution
class Solution:
def countValidWords(self, sentence: str) -> int:
def check(s: str) -> bool:
st = False
for i, c in enumerate(s):
if c.isdigit() or (c in "!.," and i < len(s) - 1):
return False
if c == "-":
if (
st
or i in (0, len(s) - 1)
or not s[i - 1].isalpha()
or not s[i + 1].isalpha()
):
return False
st = True
return True
return sum(check(s) for s in sentence.split())
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.