licenseKeyFormatting.go

/* 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:
    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.
*/

package lstring

import "strings"

func licenseKeyFormatting(S string, K int) string {
	s := strings.Replace(S, "-", "", -1)
	s = strings.ToUpper(s)
	if len(s) == 0 {
		return ""
	}

	length := len(s) + len(s)/K
	if len(s)%K == 0 {
		length--
	}
	res := make([]byte, length, length)

	for i, idx := len(s), length; i > 0; i -= K {
		if i-K >= 0 {
			copy(res[idx-K:idx], s[i-K:i])
		} else {
			copy(res[0:idx], s[0:i])
		}
		idx -= K
		if idx > 0 {
			idx--
			res[idx] = '-'
		}
	}
	return string(res)
}