## 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).