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by Sumit Chourasia | Oct 17, 2020 | Category :coding | Tags : algorithm data-structure easy leetcode string

License Key Formatting - String - Easy - LeetCode

License Key Formatting - String - Easy - LeetCode

You are given a license key represented as a string S which consists of only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes.

Given a number K, we would want to reformat the strings 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 all lowercase letters should be converted to uppercase.

Given a non-empty string S and a number K, format the string according to the rules described above.

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.
The length of string S will not exceed 12,000, and K is a positive integer.
String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
String S is non-empty.

public class Solution {
    public string LicenseKeyFormatting(string S, int K) {
        var sb = new StringBuilder();
        for(int i=S.Length-1,j=0;i>=0;i--){
                if(j!=0 && j%K==0){
        return new string(sb.ToString().ToUpper().Reverse().ToArray());

Time Complexity: O(n)

Space Complexity: O(n)