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### by Sumit Chourasia | Jan 05, 2021 | Category :coding | Tags : algorithmdata-structureeasyleetcodestring #### Reformat Phone Number - String - Easy - LeetCode

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.

``````public class Solution {
public string ReformatNumber(string number) {
number = number.Replace("-", "");
number = number.Replace(" ", "");
StringBuilder sb = new StringBuilder();
int index = 0;
int count = number.Length;
while (count > 4) { // as long as there are more than 4 left characters, we add 3 of them and a '-' for each time.
for (int i=0; i<3; i++) {
sb.Append(number[index++]);
count--;
}
sb.Append('-');
}
if (count < 4) { // less than 4 characters, add them directly.
for (int i=0; i<count; i++) {
sb.Append(number[index++]);
}
} else { // When there are exact 4 characters left, we split them by groups of size 2.
for (int i=0; i<2; i++) {
sb.Append(number[index++]);
}
sb.Append('-');
for (int i=0; i<2; i++) {
sb.Append(number[index++]);
}
}
return sb.ToString();
}
}``````

Time Complexity: O(n)

Space Complexity: O(n)