leetcode

Strong Password Checker II - Link

Question Description

A password is said to be strong if:

• It has at least 8 characters.
• It contains at least one lowercase letter.
• It contains at least one uppercase letter.
• It contains at least one digit.
• It contains at least one special character from the string “!@#$%^&*()-+”.
• It does not contain two identical characters in adjacent positions.

Given a string password, return true if it is a strong password. Otherwise, return false.

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Constraints

• 1 <= password.length <= 100
• password consists of ASCII characters.

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Approach

• Check if length is at least 8.
• Use boolean flags for lowercase, uppercase, digit, special.
• Iterate through the string, set flags and check for adjacent identical characters.
• Return true if all conditions are met.

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Dry Run

Example Input: password = “IloveLe3tcode!”

Step-by-step execution:

• Length: 13 >= 8
• Lower: ‘l’,’o’,’v’,’e’,’e’,’t’,’c’,’o’,’d’,’e’ - yes
• Upper: ‘I’,’L’,’t’,’c’ - yes
• Digit: ‘3’ - yes
• Special: ‘!’ - yes
• Adjacent: no two identical in a row
• Result: true

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Solution

class Solution {
    public boolean strongPasswordCheckerII(String password) {
        if (password.length() < 8) return false;

        boolean lower = false, upper = false, digit = false, special = false;
        String specials = "!@#$%^&*()-+";

        for (int i = 0; i < password.length(); i++) {
            char c = password.charAt(i);

            if (Character.isLowerCase(c)) lower = true;
            else if (Character.isUpperCase(c)) upper = true;
            else if (Character.isDigit(c)) digit = true;
            else if (specials.indexOf(c) >= 0) special = true;

            // check adjacent characters
            if (i > 0 && c == password.charAt(i - 1)) return false;
        }

        return lower && upper && digit && special;
    }
}

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Time and Space Complexity

• Time Complexity: O(n) — iterate through the string once
• Space Complexity: O(1) — only a few boolean flags used

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