License Key Formatting
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.
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 <= 105
s consists of English letters, digits, and dashes '-'.
1 <= k <= 104
思路一：预处理原字符串，去掉 -，然后从后面开始遍历，插入 -
public String licenseKeyFormatting(String s, int k) {
if (s == null || s.isEmpty()) return "";
StringBuilder result = new StringBuilder();
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) != '-') {
result.append(s.charAt(i));
}
}
int idx = 0;
for (int i = result.length() - 1; i > 0; i--) {
idx++;
if (idx % k == 0) {
result.insert(i, '-');
}
}
return result.toString().toUpperCase();
}