482. License Key Formatting
You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes.
Given a number K, we would want to reformat the strings 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 all lowercase letters should be converted to uppercase.
Given a non-empty string S and a number K, format the string according to the rules described above.
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.
Note:
- The length of string S will not exceed 12,000, and K is a positive integer.
- String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
- String S is non-empty.
//Time: O(n), Space: O(1) public String licenseKeyFormatting(String S, int K) { if (S == null || S.length() == 0) { return S; } S = S.replace("-", "").toUpperCase(); StringBuilder sb = new StringBuilder(); int count = 1;//开始从1开始 for (int i = S.length() - 1; i >= 0; i--) { sb.insert(0, S.charAt(i)); if (i != 0 && count % K == 0) {//注意:第一位有时虽然count % K == 0但不要加- sb.insert(0, '-'); } count++; } return sb.toString(); }