1694. 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.

Solutions

class Solution {
public:
    string reformatNumber(string number) {
        string res;
        int len = 0;
        for (auto c : number) {
            if (isdigit(c)) {
                res += c;
                if (++len == 3) {
                    res += '-';
                    len = 0;
                }
            }
        }
        int n = res.size();
        if (res[n - 1] == '-')
            res.pop_back();
        if (res[n - 2] == '-')
            swap(res[n - 3], res[n - 2]);
        return res;
    }
};