Implement strtok()

strtok takes in a string and one or more tokens, and it returns a substring which is split using all the tokens provided. The tricky part is that once you call strtok with a string, next time you can call it and pass NULL instead of any string and it will assume the string you passed in previous call.
The key steps in writing an atoi function are:

• Make a static copy of the string passed into the function for reuse on next function call.
• If new string is passed to function, make a copy of it else reuse the last sent string.
• Check for tokens in the string to split on. Make sure to keep track of end of string.
• Update the saved copy of string as well as the string passed into the function.

char* new_strtok(char* str, const char* tokens)
{
    //Making a static string to be used again on next function call.
    static char *temp;
    
    //If string passed to function is not null, copy it to our static variable
    if(str!=NULL)
    {
        temp=(char*)malloc(strlen(str));
        strcpy(temp,str);
    }

    //If the string passed is NULL and even the copy is NULL, we are done and return NULL.
    else if(temp==NULL)
    return NULL;

    //If only the string passed is NULL and the copy still has data, work with it.
    else
    {
        str=temp;
    }

    int chars=0, len = strlen(tokens), flag=0;
    
    //Run the loop till we find a token or our copy is fully parsed.
    while(*temp)
    {
        for(int i=0;i<len;i++)
        {
            if(*temp==tokens[i])
            {
                if(chars==0)
                {
                    flag=1;
                    str++;
                }
                else
                {
                    temp++;
                    str[chars]='\0';
                    return str;
                }
            }
        }
        if(flag==0)
            chars++;
        temp++;
        flag=0;
    }
    temp=NULL;
    str[chars]='\0';
    return str;
}