482. License Key Formatting

1. Question

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.

2. Examples

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.

3. Constraints

• 1 <= s.length <= 10⁵

• s consists of English letters, digits, and dashes '-'.

• 1 <= k <= 10⁴

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/license-key-formatting 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

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