Strong Password Checker

                                            
public class Solution {
    public int StrongPasswordChecker(string password) {
        int n = password.Length;
        int res = 0;
        int lower = 1, upper = 1, digit = 1;
        int[] dp = new int[n];
        
        for (int i = 0; i < n;) {
            if (char.IsLower(password[i])) lower = 0;
            if (char.IsUpper(password[i])) upper = 0;
            if (char.IsDigit(password[i])) digit = 0;
            
            int j = i;
            while (i < n && password[i] == password[j]) i++;
            dp[j] = i - j;
        }
        
        int toAdd = (lower + upper + digit);
        if (n < 6) {
            res = toAdd + Math.Max(0, 6 - (n + toAdd));
        } else {
            int overLen = Math.Max(n - 20, 0), left = 0;
            res = overLen;
            
            for (int k = 1; k < 3; k++) {
                for (int i = 0; i < n && overLen > 0; i++) {
                    if (dp[i] < 3 || dp[i] % 3 != k - 1) continue;
                    dp[i] -= Math.Min(overLen, k);
                    overLen -= k;
                }
            }
            
            for (int i = 0; i < n; i++) {
                if (dp[i] >= 3 && overLen > 0) {
                    int need = dp[i] - 2;
                    dp[i] -= overLen;
                    overLen -= need;    
                }
                if (dp[i] >= 3) {
                    left += dp[i] / 3;
                }
            }
            
            res += Math.Max(toAdd, left);
        }
        
        return res;
    }
}

                                            
func strongPasswordChecker(password string) int {
    n := len(password)
    lo, up, digit := 1, 1, 1
    missing := 3

    for _, char := range password {
        if lo > 0 && unicode.IsLower(char) {
            lo = 0
            missing--
        }
        if up > 0 && unicode.IsUpper(char) {
            up = 0
            missing--
        }
        if digit > 0 && unicode.IsDigit(char) {
            digit = 0
            missing--
        }
    }

    if n < 6 {
        return max(6-n, missing)
    }
    
    replacements := 0
    one := 0
    two := 0
    three := 0

    count := 0
    for count < n {
        start := count
        char := password[count]
        for count < n && password[count] == char {
            count++
        }
        len := count - start

        if len >= 3 {
            replacements += len / 3
            if len%3 == 0 {
                one++
            } else if len%3 == 1 {
                two++
            } else {
                three++
            }
        }
    }

    if n <= 20 {
        return max(missing, replacements)
    }

    deleteCount := n - 20
    replacements -= min(deleteCount, one)
    replacements -= min(max(deleteCount-one, 0), two*2) / 2
    replacements -= max(deleteCount-one-2*two, 0) / 3

    return deleteCount + max(missing, replacements)
}

func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

To solve this coding challenge, we need to validate and correct a password based on specific criteria to ensure it is strong. The goal is to minimize the number of steps (insertions, deletions, or replacements) required to achieve this.

# Explanation:

Password Length Requirements : The password must be at least 6 characters and at most 20 characters long.
Character Variety Requirements : The password must contain at least:

One lowercase letter
One uppercase letter
One digit

Repetition Constraint : The password must not have three consecutive repeating characters.

Steps to Solve the Problem :

Initial Checks :

Calculate the initial length of the password.
Initialize counters for lowercase, uppercase, and digit characters, starting with the assumption that none of these types are present.
Track sequences of consecutive repeating characters to handle the repetition constraint.

Analyzing Password :

Traverse the password to count lowercase, uppercase letters, digits, and detect sequences of three or more repeating characters.

Determine Required Changes :

If the length is less than 6, calculate how many characters need to be added.
If the length is more than 20, determine how many characters need to be deleted considering we might still need to make other character changes.
Calculate changes needed to satisfy character variety requirements and the avoidance of repeating characters.

Consolidate All Changes :

Sum up changes needed for each rule to derive the minimum steps needed.

Step-by-Step Explanation:

1. Initialize necessary variables

Check current password characteristics like presence of lowercase, uppercase, digits, and repetition sequences:

                                            
# Pseudocode to initialize variables:
n = length of password
lower_case_missing = 1   # Assume lowercase is missing initially
upper_case_missing = 1   # Assume uppercase is missing initially
digit_missing = 1        # Assume digit is missing initially

# Create a list to store sequences of repeating characters
repeating_sequences = []
i = 0  # Index to traverse the password

2. Traverse the password to analyze the character makeup and find repetitions

                                            
# Loop through the password to identify type of characters and sequences:
while i < n:
    if 'a' <= password[i] <= 'z':  
        lower_case_missing = 0  # Found at least one lowercase
    if 'A' <= password[i] <= 'Z':
        upper_case_missing = 0  # Found at least one uppercase
    if '0' <= password[i] <= '9':
        digit_missing = 0       # Found at least one digit

    # Detecting repeating sequences
    start = i
    while i < n and password[i] == password[start]:
        i += 1
    
    length_of_sequence = i - start
    if length_of_sequence >= 3:
        repeating_sequences.append(length_of_sequence)

3. Determine number of additions or deletions needed

Add/delete based on the password length and required characters:

                                            
# Calculate missing char types:
missing_types = lower_case_missing + upper_case_missing + digit_missing

# Initialization for steps needed
steps_needed = 0

# For a short password less than 6:
if n < 6:
    steps_needed = max(missing_types, 6 - n)

# For a long password greater than 20:
elif n > 20:
    over_len = n - 20
    steps_needed += over_len

    # Modify the repeating sequences to reduce the length first
    for k in range(1, 3):
        for j in range(len(repeating_sequences)):
            if over_len <= 0:
                break
            if repeating_sequences[j] >= 3 and repeating_sequences[j] % 3 == (k - 1):
                removal = min(over_len, k)
                repeating_sequences[j] -= removal
                over_len -= removal

    # Further reduce sequences that are still long
    for i in range(len(repeating_sequences)):
        if repeating_sequences[i] >= 3:
            steps_needed += repeating_sequences[i] // 3

    steps_needed += max(missing_types, steps_needed)

else:
    # For passwords with length between 6 and 20:
    for seq_len in repeating_sequences:
        steps_needed += seq_len // 3
    steps_needed += max(missing_types, steps_needed)

4. Return the total number of steps calculated

                                            
# Final steps needed to correct the password:
return steps_needed

The above pseudocode provides an organized and detailed approach to identifying the minimum operations needed to ensure a password meets the predefined strong password criteria. Each step is systematically broken down and explained clearly.