题目大意：把序列号格式化，每组要求是K个字符（第一组 <= K），组和组之间用“-”分隔。

https://leetcode.com/problems/license-key-formatting/description/

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:

1. The length of string S will not exceed 12,000, and K is a positive integer.
2. String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
3. String S is non-empty.

Solution

Time complexity: O(n)

Space complexity: O(n)

C++

// Author: Huahua
// Running time: 16 ms
class Solution {
public:
    string licenseKeyFormatting(string S, int K) {
      string s;
      for (char c : S)
        if (c != '-') s.push_back(toupper(c));
      int first = s.length() % K;
      string ans = s.substr(0, first);
      for (int i = 0; i < s.length() - first; ++i) {
        if (i % K == 0 && (i + first)) ans += '-';
        ans.push_back(s[i + first]);
      }
      return ans;
    }
};

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