1694. Reformat Phone Number #

Problem #

You are given a phone number as a string number. number consists of digits, spaces ' ', and/or dashes '-'.

You would like to reformat the phone number in a certain manner. Firstly, remove all spaces and dashes. Then, group the digits from left to right into blocks of length 3 until there are 4 or fewer digits. The final digits are then grouped as follows:

2 digits: A single block of length 2.
3 digits: A single block of length 3.
4 digits: Two blocks of length 2 each.

The blocks are then joined by dashes. Notice that the reformatting process should never produce any blocks of length 1 and produce at most two blocks of length 2.

Return the phone number after formatting.

Example 1:

Input: number = "1-23-45 6"
Output: "123-456"
Explanation: The digits are "123456".
Step 1: There are more than 4 digits, so group the next 3 digits. The 1st block is "123".
Step 2: There are 3 digits remaining, so put them in a single block of length 3. The 2nd block is "456".
Joining the blocks gives "123-456".

Example 2:

Input: number = "123 4-567"
Output: "123-45-67"
Explanation: The digits are "1234567".
Step 1: There are more than 4 digits, so group the next 3 digits. The 1st block is "123".
Step 2: There are 4 digits left, so split them into two blocks of length 2. The blocks are "45" and "67".
Joining the blocks gives "123-45-67".

Example 3:

Input: number = "123 4-5678"
Output: "123-456-78"
Explanation: The digits are "12345678".
Step 1: The 1st block is "123".
Step 2: The 2nd block is "456".
Step 3: There are 2 digits left, so put them in a single block of length 2. The 3rd block is "78".
Joining the blocks gives "123-456-78".

Example 4:

Input: number = "12"
Output: "12"

Example 5:

Input: number = "--17-5 229 35-39475 "
Output: "175-229-353-94-75"

Constraints:

2 <= number.length <= 100
number consists of digits and the characters '-' and ' '.
There are at least two digits in number.

Problem Summary #

You are given a phone number number as a string. number consists of digits, spaces ' ', and dashes '-'.

Please reformat the phone number in the following way.

First, delete all spaces and dashes.
Then, split the digits from left to right into blocks of 3 digits each until 4 or fewer digits remain. The remaining digits are then split according to the following rules:
- 2 digits: a single block containing 2 digits.
- 3 digits: a single block containing 3 digits.
- 4 digits: two blocks each containing 2 digits.

Finally, connect these blocks with dashes. Note that during the reformatting process, no block containing only 1 digit should be generated, and at most two blocks containing 2 digits should be generated. Return the formatted phone number.

Solution Approach #

Easy problem. First determine whether the number has 2 or 4 digits; if so, output these two cases separately. The remaining cases all have more than 3 digits. Take the remainder and determine whether the remaining digits are 2 or 4. At this point, it is impossible to have 1 digit remaining. A remainder of 1 when divided by 3 means there are 4 digits remaining, so the last 4 digits need to be output in groups of 2. A remainder of 2 when divided by 3 means there are 2 digits remaining. After handling the end, connect the preceding digits in reverse order in groups of 3 with “-”. The cases that may need attention can be found in the test file.

Code #

package leetcode

import (
	"strings"
)

func reformatNumber(number string) string {
	parts, nums := []string{}, []rune{}
	for _, r := range number {
		if r != '-' && r != ' ' {
			nums = append(nums, r)
		}
	}
	threeDigits, twoDigits := len(nums)/3, 0
	switch len(nums) % 3 {
	case 1:
		threeDigits--
		twoDigits = 2
	case 2:
		twoDigits = 1
	default:
		twoDigits = 0
	}
	for i := 0; i < threeDigits; i++ {
		s := ""
		s += string(nums[0:3])
		nums = nums[3:]
		parts = append(parts, s)
	}
	for i := 0; i < twoDigits; i++ {
		s := ""
		s += string(nums[0:2])
		nums = nums[2:]
		parts = append(parts, s)
	}
	return strings.Join(parts, "-")
}