练习 6-4 编写一个程序，根据单词的出现频率按降序打印输入的各个不同单词，并在 每个单词的前面标上它的出现次数

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>

#define MAXWORD 100

/*Author Stat:  founded 2017 11 26*/
static size_t word_count = 0;

struct line{
   char *word;
   size_t count;
   struct line *next;
};


int getword(char *,int);
struct line *additem(struct line *,char *);
void swapcontent(struct line *,struct line *);
void showitem(struct line *);
struct line *talloc(void);
char *strdup(char *);


int main(void)
{
   struct line *head;
   char word[MAXWORD];


   head = NULL;
   while(getword(word,MAXWORD) != EOF)
      if(isalpha(word[0]))
         head = additem(head,word);
   printf("Total word: %lu\n",word_count);
   showitem(head);
   return 0;
}


int getword(char *word,int n)
{
   int c,getch(void);
   void ungetch(int);
   char *w = word;


   while(isspace(c = getch()))
      ;
   if(c != EOF)
      *w++ = c;
   if(!isalpha(c)){
      *w = '\0';
 return c;
   }
   for(;--n > 0; w++)
      if(!isalpha(*w = getch())){
         ungetch(*w);
break;
      }
   *w = '\0';
   return word[0];
}


struct line *additem(struct line *p,char *w)
{
   if(p ==  NULL){
      if((p = talloc()) == NULL){
    printf("Out of memory!\n");
exit(EXIT_FAILURE);
 }
      if((p->word = strdup(w)) == NULL){
    printf("Out of memory!\n");
    exit(EXIT_FAILURE);
 }
      p->count = 1;
      p->next = NULL;
      word_count++;
   }else if(strcmp(p->word,w) == 0)
      p->count++;
   else
      p->next = additem(p->next,w);
   if(p->next != NULL)
      swapcontent(p,p->next);
   return p;
}


void swapcontent(struct line *p,struct line *pn)
{
    size_t temp;
char *temps;


if(p->count < pn->count){
  temp = p->count;
  p->count = pn->count;
  pn->count = temp;
  temps = p->word;
  p->word = pn->word;
       pn->word = temps;
    }


}


void showitem(struct line *p)
{
   if(p != NULL){
     printf("%4lu %s\n",p->count,p->word);
showitem(p->next);
free(p->word);
p->word = NULL;
free(p);
p = NULL;
   }
}


struct line *talloc(void)
{
   return (struct line *)malloc(sizeof(struct line));
}


char *strdup(char *s)
{
   char *p;


   if((p = (char *)malloc(strlen(s)+1)) != NULL)
      strcpy(p,s);
   return p;
}


#define BUFSIZE 100


static char buf[BUFSIZE];
static int ps = 0;


int getch(void)
{
   return (ps > 0) ? buf[--ps] : getchar();
}

void ungetch(int c)
{
   if(ps > BUFSIZE)
      printf("stack too full!\n");
   else
      buf[ps++] = c;

}