482_license_key_formatting.java

/* 
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.
*/ 

class Solution {
    public String licenseKeyFormatting(String S, int K) {
        
        StringBuilder sb = new StringBuilder(); 
        
        int index = S.length() - 1, k = K; 
        while (index != -1) {
            char c = S.charAt(index); 
            if (c != '-') { 
                if (k == 0) {
                    sb.append('-'); 
                    k = K; 
                } 
                
                sb.append(c); 
                k --; 
            }
            index --; 
        } 
        
        StringBuilder res = new StringBuilder(); 
        for (int i = sb.length() - 1;i >= 0;i --) {
            res.append(sb.charAt(i)); 
        } 
        
        return res.toString().toUpperCase(); 
    }
}