// You are given a license key represented as a string S which consists 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.
// Note:
// 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.

// Solution 1: 
// check from end of S
// remove the first "-" if there it is
// convert lowercase char to uppercase char
// 
// Time complexity: O(n) n is S.count
// Space comlexity: O(m) m is the returned string.count which should < n
class Solution {
    func licenseKeyFormatting(_ S: String, _ K: Int) -> String {
        var formatted = String()
        
        var kindex = 0
        for i in S.indices.reversed() {
            guard S[i] != "-" else { continue }
            formatted.insert(S[i], at: formatted.startIndex)
            
            kindex += 1
            if kindex == K {
                // add dash
                formatted.insert("-", at: formatted.startIndex)
                kindex = 0
            }
        }
        
        // remove the first "-" if needed
        if formatted.first == "-" { formatted.removeFirst() }
        // convert lower char to uppercased char
        return String(formatted).uppercased()
    }
    
    // check and return the converted chars
    // a-z -> A-Z
    // A-Z -> keep
    // 0-9 -> keep
    // use char.uppercased()
}