Password Strength - Solution & Explanation

Problem Overview: You are given a password string and must determine whether it satisfies common strength requirements such as minimum length and the presence of different character categories (uppercase, lowercase, digits, or special characters). The task reduces to scanning the string and verifying that all required constraints are satisfied.

Approach 1: Direct Rule Checking (Brute Force) (Time: O(n), Space: O(1))

The simplest method checks each rule independently. For example, run separate loops to verify whether the password contains at least one lowercase letter, one uppercase letter, one digit, and optionally a special character. Each loop iterates through the string and sets a flag when the rule is satisfied. While this approach is straightforward, it may traverse the string multiple times. The logic is easy to reason about and works well for short inputs, but it is inefficient compared to a single-pass solution.

Approach 2: Single Pass Character Classification (Time: O(n), Space: O(1))

A more efficient method scans the string once and classifies each character during the iteration. Maintain boolean flags such as hasLower, hasUpper, hasDigit, and hasSpecial. For each character, check its category using ASCII checks or helper functions. After the iteration finishes, combine the flags with the minimum length requirement to determine whether the password is strong. This avoids repeated traversal and is the typical solution used in interviews when working with strings.

Approach 3: Bitmask Optimization (Time: O(n), Space: O(1))

You can compress the category checks into a small integer bitmask. Assign one bit per requirement (for example: lowercase = 1, uppercase = 2, digit = 4, special = 8). While iterating through the password, set the corresponding bit whenever a character matches a category. At the end, compare the mask against the required bit pattern. This approach is still O(n) but slightly cleaner when handling multiple rules and scales well if more constraints are introduced. Bitmasks are a common technique in bit manipulation problems and allow constant‑time checks of multiple conditions.

Recommended for interviews: The single-pass classification approach is what interviewers usually expect. It demonstrates efficient iteration over a string and constant-space state tracking. Mentioning the brute-force multi-pass method shows you considered simpler solutions first, but implementing the O(n) single traversal highlights stronger problem-solving and code efficiency.