482.LicenseKeyFormatting.cs

// 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

public class Solution {
    public string LicenseKeyFormatting(string s, int k) {
        s = s.Replace("-", "").ToUpper();
        //intution: insert '-' from the end of the string with k interval.
        var sb = new StringBuilder(s);
        for(int i = s.Length - k; i > 0; i -= k){
            sb.Insert(i, '-');
        }
        return sb.ToString();
    }
}


public class Solution {
    public string LicenseKeyFormatting(string s, int k) {
        
        s = s.Replace("-", "").ToUpper();
        string res = "";
        for(int i = s.Length; i >= 0; i -= k){
            string temp = "";
            for(int j = Math.Max(i - k, 0); j < i; j++ ){
                temp += s[j];
            }
            res = temp + res;
            if(i - k > 0)
                res = "-" + res;
        }
        return res;
    }
}