# LeetCode 1694. Reformat Phone Number

## Description

https://leetcode.com/problems/reformat-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`.

## Explanation

We can record the numbers first, then keep grouping 3 numbers in a group until less than 4 numbers. If 4 numbers left, divide to 2 groups, otherwise group to 1 group.

## Python Solution

``````class Solution:
def reformatNumber(self, number: str) -> str:
number_list = []

for c in number:
if c.isdigit():
number_list.append(c)

start = 0

groups = defaultdict(list)

while len(number_list) > 4:
size = len(groups)
for i in range(3):
groups[size].append(number_list.pop(0))

if len(number_list) == 4:
size = len(groups)
for i in range(2):
groups[size].append(number_list.pop(0))

size = len(groups)

for i in range(2):
groups[size].append(number_list.pop(0))

else:
size = len(groups)

for i in range(len(number_list)):
groups[size].append(number_list.pop(0))

result = []
for key, value in groups.items():
result.append("".join(value))

return "-".join(result)

``````
• Time Complexity: O(N).
• Space Complexity: O(N).