Reformat Phone Number — C++

// Time:  O(n)
// Space: O(1)

// inplace solution
class Solution {
public:
    string reformatNumber(string number) {
        int src_len = 0;
        for (const auto& c : number) {  // remove non-digit characters
            if (isdigit(c)) {
                number[src_len++] = c;
            }
        }
        int dst_len = src_len + (src_len - 1) / 3;
        number.resize(dst_len);  // resize the buffer to expected final size
        for (int i = src_len - 1, curr = dst_len - 1, l = (3 - src_len % 3) % 3; i >= 0; --i, ++l) {
            if (l && l % 3 == 0) {  // group by 3 digits
                number[curr--] = '-';
            }
            number[curr--] = number[i];
        }
        if (dst_len >= 3 && number[dst_len - 2] == '-') {  // adjust for the 4 digits case
            swap(number[dst_len - 3], number[dst_len - 2]);
        }
        return number;
    }
};