代码如下：

#include<iostream>
#include<string>
#include<fstream>
#include<map>
#include<iomanip>
#include<algorithm>
#include<vector>

int read_file(std::map<std::string,int> &map_words)
{
	std::string filename = "words.txt";
	std::fstream fin;
	fin.open(filename.c_str(), std::ios::in);

	std::string tmp;

	while (fin >> tmp)
	{
		++map_words[tmp];
	}

	return 0;
}

int save_words(std::multimap<int, std::string, std::less<int>> &multimap_words, std::map<std::string,int> map_words)
{
	for(std::map<std::string, int>::iterator iter = map_words.begin(); iter != map_words.end(); ++iter)
{
	multimap_words.insert(std::pair<int, std::string>(iter->second, iter->first));
}

	return 0;
}

int print_words(std::multimap<int, std::string, std::less<int>> multimap_words)
{
	int i = 0;
	for(std::multimap<int, std::string>::iterator iter = multimap_words.begin(); (iter != multimap_words.end()); ++iter, ++i)
	{
		if((iter->first == prev(iter,1)->first) || (i < 20)  || 1)
		{
			std::cout << "第" << i << "个单词,它出现的频次是：" << iter->first << " 它是：" << iter->second << std::endl;
		}
		else
		{
			break;
		}
	}
	return 0;
}

int main(int argc, char* argv[])
{
	std::map<std::string,int> map_words;
	read_file(map_words);
	std::multimap<int, std::string, std::less<int>> multimap_words;
	save_words(multimap_words,map_words);

	print_words(multimap_words);

	return 0;
}

运行程序的结果如下：