Leetcode #193: Valid Phone Numbers
In this guide, we solve Leetcode #193 Valid Phone Numbers 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
Given a text file file.txt that contains a list of phone numbers (one per line), write a one-liner bash script to print all valid phone numbers. You may assume that a valid phone number must appear in one of the following two formats: (xxx) xxx-xxxx or xxx-xxx-xxxx.
Quick Facts
- Difficulty: Easy
- Premium: No
- Tags: Shell
Intuition
The constraints allow a direct scan that keeps only the essential state.
By translating the requirements into a clean loop, the logic stays easy to reason about.
Approach
Iterate through the data once, updating the state needed to compute the answer.
Return the final state after the traversal is complete.
Steps:
- Parse the input.
- Iterate and update state.
- Return the computed answer.
Example
987-123-4567
123 456 7890
(123) 456-7890
Python Solution
import re
def print_valid_numbers(path: str = "file.txt") -> None:
pattern = re.compile(r"^(\(\d{3}\) |\d{3}-)\d{3}-\d{4}$")
with open(path, "r", encoding="utf-8") as f:
for line in f:
line = line.rstrip("\n")
if pattern.match(line):
print(line)
Complexity
The time complexity is O(n). The space complexity is O(1) to O(n).
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.