字符串逆置

#include <iostream>
#include <string>
#include <vector>
#include <stdlib.h>
#include <assert.h>
int count_word(const std::string&);
void swap(char&, char&);
void reverse_string(std::string &s, std::size_t begin, std::size_t end);
std::string& reverse_word(std::string &s);
void print(const std::string &s);
int main(int argc, char** argv)
{
    std::string s =
            "I have a book named Advanced Programming of UNIX Environment";
    reverse_word(s);
    print(s);
    return 0;
}
void print(const std::string &s)
{
    std::cout << s << std::endl;
}
void swap(char &a, char &b)
{
    char _tmp = a;
    a = b;
    b = _tmp;
}
void reverse_string(std::string &s, std::size_t begin, std::size_t end)
{
    assert(begin <= end);
    while (begin < end)
    {
        swap(s[begin++], s[end--]);
    }
}
std::string& reverse_word(std::string &s)
{
    bool flag = false;
    std::size_t first = 0;
    for (std::size_t ix = 0; ix != s.length(); ++ix)
    {
        if (s[ix] == ' ' && flag)
        {
            flag = false;
            reverse_string(s, first, ix - 1);
        }
        else if (!flag && isalpha(s[ix]))
        {
            flag = true;
            first = ix;
        }
        else if (ix == s.length() - 1 && flag)
        {
            flag = false;
            reverse_string(s, first, ix);
        }
    }
    reverse_string(s, 0, s.length() - 1);
    return s;
}
int count_word(const std::string &s)
{
    bool flag = false;
    std::size_t cnt = 0;
    std::vector<std::string> vec;
    std::size_t first = 0;
    for (std::size_t ix = 0; ix != s.length(); ++ix)
    {
        if (s[ix] == ' ' && flag)
        {
            flag = false;
            std::string tmp_s = s.substr(first, ix - first);
            vec.push_back(tmp_s);
            cnt++;
        }
        else if (!flag && isalnum(s[ix]))
        {
            flag = true;
            first = ix;
        }
        else if (ix == s.length() - 1 && flag)
        {
            std::string tmp_s = s.substr(first, ix - first + 1);
            vec.push_back(tmp_s);
            cnt++;
        }
    }
    std::vector<std::string>::const_iterator iter = vec.begin();
    for (; iter != vec.end(); iter++)
    {
        std::cout << *iter << std::endl;
    }
    return cnt;
}