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## Problem

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.

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 <= 10^5
• s consists of English letters, digits, and dashes ‘-‘.
• 1 <= k <= 10^4

## Code

class Solution {
public String licenseKeyFormatting(String s, int k) {
StringBuilder sb = new StringBuilder();

int count = k;
int index = s.length() - 1;
while(index >= 0) {
char c = s.charAt(index--);

if(c == '-') continue;

if(count != 0) {
sb.append(c);
count--;
} else {
sb.append('-');
sb.append(c);
count = k - 1;
}
}

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