一道笔试题的解法

题目是：

程序为：

#include<iostream>
#include<string>
using namespace std;

string format(string macstr)
{
    string formatedstr = "";
    for(int i = 0; i < macstr.size(); i++)
    {
        char c = macstr[i];
            if((c >= 'a' && c <= 'z') 
            || (c >= 'A' && c <= 'Z')
            || (c >= '0' && c <= '9'))
            {
            formatedstr.append(1,c);
        }
        }
    for(int j = 2; j < formatedstr.size(); j += 3)
    {
        
        formatedstr.insert(j,1,':');
            
    }
    return formatedstr;
};

int main()
{
    string s = "00:50-8d-49-db";
    cout << "formated: " << format(s) << endl; 
    
    s = "00:508d-49-db";
    cout << "formated: " << format(s) << endl; 

    return 1;
}