leetcode-482-easy

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();
}