Is Strong Password Checker easy or hard?

Strong Password Checker is rated Hard because it combines multiple constraints: length limits, character type requirements, and repetition rules. The challenge is designing a greedy strategy that minimizes edits while handling all cases efficiently.

Strong Password Checker Python/Java solution

Python and Java implementations typically follow the greedy replacement strategy. The code scans the string, counts repeating groups, and applies insert, delete, or replace operations depending on length and missing character types. Both implementations achieve O(n) time complexity.

How to solve Strong Password Checker in O(n)?

Traverse the password once and track three values: missing character types, lengths of repeating segments, and total length. Use greedy rules to determine how many insertions, deletions, or replacements are needed. When length exceeds 20, apply deletions to repeating sequences first to minimize future replacements.

What is the best approach for Strong Password Checker?

The best approach is a greedy strategy that tracks missing character types, repeating sequences, and password length in one pass. Replacements handle repeating characters, insertions fix short passwords, and deletions reduce length and repetition in long passwords. This solution runs in O(n) time and O(1) space.

Is Strong Password Checker asked at Google/Amazon/Meta?

Strong Password Checker is considered a high-difficulty string and greedy optimization problem similar to questions asked at companies like Google and Meta. It tests constraint handling, greedy reasoning, and careful case analysis, which are common interview themes.

What data structure is used in Strong Password Checker?

Most optimal implementations only use counters and simple variables, making it primarily a greedy string problem. Some variations use a priority queue (heap) to prioritize deletions in repeating segments, but this is optional and not required for the O(n) solution.

What is the time complexity of Strong Password Checker?

The optimal solution runs in O(n) time where n is the password length. The algorithm scans the string once to detect repeating sequences and missing character categories. Space complexity is O(1) because only counters and small variables are maintained.

Description Solution Video 59:22

420. Strong Password Checker

Hard16.7% acceptance7 companies

A password is considered strong if the below conditions are all met:

It has at least 6 characters and at most 20 characters.
It contains at least one lowercase letter, at least one uppercase letter, and at least one digit.
It does not contain three repeating characters in a row (i.e., "Baaabb0" is weak, but "Baaba0" is strong).