482. License Key Formatting

You are given a license key represented as a string s that consists of only alphanumeric characters and dashes. The string is separated into n + 1 groups by n dashes. You are also given an integer k.

We want to reformat the string s such that each group contains exactly k characters, except for the first group, which could be shorter than k but still must contain at least one character. Furthermore, there must be a dash inserted between two groups, and you should convert all lowercase letters to uppercase.

Return the reformatted license key.

Example 1:

Input: s = "5F3Z-2e-9-w", k = 4
Output: "5F3Z-2E9W"
Explanation: The string s has been split into two parts, each part has 4 characters.
Note that the two extra dashes are not needed and can be removed.

Example 2:

Input: s = "2-5g-3-J", k = 2
Output: "2-5G-3J"
Explanation: The string s has been split into three parts, each part has 2 characters except the first part as it could be shorter as mentioned above.

Constraints:

• 1 <= s.length <= 105
• s consists of English letters, digits, and dashes '-'.
• 1 <= k <= 104

Solution:

class Solution {
    public String licenseKeyFormatting(String s, int k) {
        StringBuilder sb = new StringBuilder();
        int j = 0;
        for (int i = s.length() - 1; i >= 0; i--){
            char ch = s.charAt(i);
            if (ch != '-'){
                if (j == k){
                    j = 0;
                    sb.append('-');
                }

                ch = Character.toUpperCase(ch);

                sb.append(ch);
                j++;
            }
        }

        return sb.reverse().toString();
    }
}

// TC: O(n)
// SC: O(1) We are not using any extra space other than the output string.

class Solution {
    public String licenseKeyFormatting(String s, int k) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++){
            if (s.charAt(i) != '-'){
                char c = Character.toUpperCase(s.charAt(i));
                sb.append(c);
            }
        }

        StringBuilder sb2 = new StringBuilder();
        int i = sb.length() - 1;
        /*  01234
            2 5g - 3j
            i
            k = 2

        */

        /*  01234567
            5F3Z-2e9w
               i 

            k = 4
        */
        while(i >= k - 1){ // 2-1 =1 // 4-1=3
            int count = k;
            while(count > 0){
                sb2.append(sb.charAt(i));
                i--;
                count--;
            }
            if (i >= 0){
                sb2.append('-');
            }


        }

        while(i >= 0){
            sb2.append(sb.charAt(i));
            i--;
        }

        sb2.reverse();

        return sb2.toString();
    }
}

// TC: O(n)
// SC: O(n)