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.

Solution：遍歷

思路：

Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

class Solution {
    public String licenseKeyFormatting(String s, int k) {
        StringBuilder sb = new StringBuilder();
        for (int i = s.length() - 1; i >= 0; i--)
            if (s.charAt(i) != '-')
                sb.append(sb.length() % (k + 1) == k ? '-' : "").append(s.charAt(i));
        return sb.reverse().toString().toUpperCase();
    } 
}