Algorithm to Reformat the Phone Number

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.

Hints:
Discard all the spaces and dashes.
Use a while loop. While the string still has digits, check its length and see which rule to apply.

Phone Number Reformat Algorithm

First, we need to remove all spaces and dashes. Then we can group the digits (into 3) while there are at least 5 digits. Then, we can apply the remaining rules. Using Python gives and advantage to apply the string slicing easily to obtain the substrings.

1
2
3
4
5
6
7
8
9
10
11
12
13

class Solution:
    def reformatNumber(self, number: str) -> str:
        s = number.replace(' ', '').replace('-', '')
        i, j = 0, len(s)
        ans = ""
        while i + 4 < j:
            ans += s[i: i + 3] + "-"
            i += 3
        if i + 4 == j:
            ans += s[i: i + 2] + "-" + s[i + 2:]
        else:
            ans += s[i:]
        return ans

class Solution:
    def reformatNumber(self, number: str) -> str:
        s = number.replace(' ', '').replace('-', '')
        i, j = 0, len(s)
        ans = ""
        while i + 4 < j:
            ans += s[i: i + 3] + "-"
            i += 3
        if i + 4 == j:
            ans += s[i: i + 2] + "-" + s[i + 2:]
        else:
            ans += s[i:]
        return ans

The time complexity of the above Phone Number String format algorithm is O(N). The space complexity is O(N) as we are allocating space to store the formatted phone number string.

–EOF (The Ultimate Computing & Technology Blog) —