void dldelete(
  struct address *i, /* item to delete */
  struct address **start,  /* first item */
  struct address **last) /* last item */
{
  if(i->prior) i->prior->next = i->next;
  else { /* new first item */
    *start = i->next;
    if(start) start->prior = NULL;
  }

  if(i->next) i->next->prior = i->prior;
  else   /* deleting last element */
    *last = i->prior;
}

listing 16
/* A simple mailing list program that illustrates the
   use and maintenance of doubly linked lists.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct address {
  char name[30];
  char street[40];
  char city[20];
  char state[3];
  char zip[11]; 
  struct address *next;  /* pointer to next entry */
  struct address *prior;  /* pointer to previous record */
};

struct address *start;  /* pointer to first entry in list */
struct address *last;  /* pointer to last entry */
struct address *find(char *);

void enter(void), search(void), save(void);
void load(void), list(void);
void mldelete(struct address **, struct address **);
void dls_store(struct address *i, struct address **start,
               struct address **last);
void inputs(char *, char *, int), display(struct address *);
int menu_select(void);

int main(void)
{
  start = last = NULL;  /* initialize start and end pointers */

  for(;;) {
    switch(menu_select()) {
      case 1: enter(); /* enter an address */
        break;
      case 2: mldelete(&start, &last); /* remove an address */
        break;
      case 3: list(); /* display the list */
        break;
      case 4: search(); /* find an address */
        break;
      case 5: save();  /* save list to disk */
        break;
      case 6: load();  /* read from disk */
        break;
      case 7: exit(0);
    }
  }
  return 0;
}

/* Select an operation. */
int menu_select(void)
{
  char s[80];
  int c;

  printf("1. Enter a name\n");
  printf("2. Delete a name\n");
  printf("3. List the file\n");
  printf("4. Search\n");
  printf("5. Save the file\n");
  printf("6. Load the file\n");
  printf("7. Quit\n");
  do {
    printf("\nEnter your choice: ");
    gets(s);
    c = atoi(s);
  } while(c<0 || c>7);
  return c;
}

/* Enter names and addresses. */
void enter(void)
{
  struct address *info;

  for(;;) {
    info = (struct address *)malloc(sizeof(struct address));
    if(!info) {
      printf("\nout of memory");
      return;
    }

    inputs("Enter name: ", info->name, 30);
    if(!info->name[0]) break;  /* stop entering */
    inputs("Enter street: ", info->street, 40);
    inputs("Enter city: ", info->city, 20);
    inputs("Enter state: ", info->state, 3);
    inputs("Enter zip: ", info->zip, 10);

    dls_store(info, &start, &last);
  } /* entry loop */
}

/* This function will input a string up to
   the length in count and will prevent
   the string from being overrun.  It will also
   display a prompting message. */
void inputs(char *prompt, char *s, int count)
{
  char p[255];

  do {
    printf(prompt);
    fgets(p, 254, stdin);
    if(strlen(p) > count) printf("\nToo Long\n");
  } while(strlen(p) > count);

  p[strlen(p)-1] = 0; /* remove newline character */
  strcpy(s, p);
}

/* Create a doubly linked list in sorted order. */
void dls_store(
  struct address *i,   /* new element */
  struct address **start, /* first element in list */
  struct address **last /* last element in list */
)
{
  struct address *old, *p;

  if(*last==NULL) {  /* first element in list */
    i->next = NULL;
    i->prior = NULL;
    *last = i;
    *start = i;
    return;
  }
  p = *start; /* start at top of list */

  old = NULL;
  while(p) {
    if(strcmp(p->name, i->name)<0){
      old = p;
      p = p->next;
    }
    else {
      if(p->prior) {
        p->prior->next = i;
        i->next = p;
        i->prior = p->prior;
        p->prior = i;
        return;
      }
      i->next = p; /* new first element */
      i->prior = NULL;
      p->prior = i;
      *start = i;
      return;
    }
  }
  old->next = i; /* put on end */
  i->next = NULL;
  i->prior = old;
  *last = i;
}

/* Remove an element from the list. */
void mldelete(struct address **start, struct address **last)
{
  struct address *info;
  char s[80];

  inputs("Enter name: ", s, 30);
  info = find(s);
  if(info) {
    if(*start==info) {
      *start=info->next;
      if(*start) (*start)->prior = NULL;
      else *last = NULL;
    }
    else {
      info->prior->next = info->next;
      if(info!=*last)
          info->next->prior = info->prior;
      else
        *last = info->prior;
    }
    free(info);  /* return memory to system */
  }
}

/* Find an address. */
struct address *find( char *name)
{
  struct address *info;

  info = start;
  while(info) {
    if(!strcmp(name, info->name)) return info;
    info = info->next;  /* get next address */
  }
  printf("Name not found.\n");
  return NULL;  /* not found */
}

/* Display the entire list. */
void list(void)
{
  struct address *info;

  info = start;
  while(info) {
    display(info);
    info = info->next;  /* get next address */
  }
  printf("\n\n");
}

/* This function actually prints the fields in each address. */
void display(struct address *info)
{
    printf("%s\n", info->name);
    printf("%s\n", info->street);
    printf("%s\n", info->city);
    printf("%s\n", info->state);
    printf("%s\n", info->zip);
    printf("\n\n");
}

/* Look for a name in the list. */
void search(void)
{
  char name[40];
  struct address *info;

  printf("Enter name to find: ");
  gets(name);
  info = find(name);
  if(!info) printf("Not Found\n");
  else display(info);
}

/* Save the file to disk. */
void save(void)
{
  struct address *info;

  FILE *fp;

  fp = fopen("mlist", "wb");
  if(!fp) {
    printf("Cannot open file.\n");
    exit(1);
  }
  printf("\nSaving File\n");

  info = start;
  while(info) {
    fwrite(info, sizeof(struct address), 1, fp);
    info = info->next;  /* get next address */
  }
  fclose(fp);
}

/* Load the address file. */
void load()
{
  struct address *info;
  FILE *fp;

  fp = fopen("mlist", "rb");
  if(!fp) {
    printf("Cannot open file.\n");
    exit(1);
  }

  /* free any previously allocated memory */
  while(start) {
    info = start->next;
    free(info);
    start = info;
  }

  /* reset top and bottom pointers */
  start = last = NULL;

  printf("\nLoading File\n");
  while(!feof(fp)) {
    info = (struct address *) malloc(sizeof(struct address));
    if(!info) {
      printf("Out of Memory");
      return;
    }
    if(1 != fread(info, sizeof(struct address), 1, fp)) break;
    dls_store(info, &start, &last);
  }
  fclose(fp);
}

listing 17
struct tree {
  char info;
  struct tree *left;
  struct tree *right;
};

struct tree *stree(
  struct tree *root,
  struct tree *r,
  char info)
{
  if(!r) {
    r = (struct tree *) malloc(sizeof(struct tree));
    if(!r) {
      printf("Out of Memory\n");
      exit(0);
    }
    r->left = NULL;
    r->right = NULL;
    r->info = info;
    if(!root) return r; /* first entry */
    if(info < root->info) root->left = r;
    else root->right = r;
    return r;
  }
  if(info < r->info)
    stree(r,r->left,info);
  else
    stree(r,r->right,info);

  return root; 
}

listing 18
void inorder(struct tree *root)
{
  if(!root) return;

  inorder(root->left);
  if(root->info) printf("%c ", root->info);
  inorder(root->right);
}

listing 19
void preorder(struct tree *root)
{
  if(!root) return;

  if(root->info) printf("%c ", root->info);
  preorder(root->left);
  preorder(root->right);
}

void postorder(struct tree *root)
{
  if(!root) return;

  postorder(root->left);
  postorder(root->right);
  if(root->info) printf("%c ", root->info);
}

listing 20
void print_tree(struct tree *r, int l)
{
  int i;

  if(r == NULL) return;

  print_tree(r->right, l+1);
  for(i=0; i<l; ++i) printf(" ");
  printf("%c\n", r->info);
  print_tree(r->left, l+1);
}

listing 21
/* This program displays a binary tree. */

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

struct tree {
  char info;
  struct tree *left;
  struct tree *right;
};

struct tree *root; /* first node in tree */
struct tree *stree(struct tree *root,
                   struct tree *r, char info);
void print_tree(struct tree *root, int l);

int main(void)
{
  char s[80];

  root = NULL;  /* initialize the root */

  do {
    printf("Enter a letter: ");
    gets(s);
    root = stree(root, root, *s);
  } while(*s);

  print_tree(root, 0);

  return 0;
}

struct tree *stree(
  struct tree *root,
  struct tree *r,
  char info)
{

  if(!r) {
    r = (struct tree *) malloc(sizeof(struct tree));
    if(!r) {
      printf("Out of Memory\n");
      exit(0);
    }
    r->left = NULL;
    r->right = NULL;
    r->info = info;
    if(!root) return r; /* first entry */
    if(info < root->info) root->left = r;
    else root->right = r;
    return r;
  }

  if(info < r->info)
    stree(r, r->left, info);
  else
    stree(r, r->right, info);

  return root;
}

void print_tree(struct tree *r, int l)
{
  int i;

  if(!r) return;

  print_tree(r->right, l+1);
  for(i=0; i<l; ++i) printf(" ");
  printf("%c\n", r->info);
  print_tree(r->left, l+1);
}

listing 22
struct tree *search_tree(struct tree *root, char key)
{
  if(!root) return root;  /* empty tree */
  while(root->info != key) {
    if(key<root->info) root = root->left;
    else root = root->right;
    if(root == NULL) break;
  }
  return root;
}

listing 23
struct tree *dtree(struct tree *root, char key)
{
  struct tree *p,*p2;

  if(!root) return root; /* not found */

  if(root->info == key) { /* delete root */
    /* this means an empty tree */
    if(root->left == root->right){
      free(root);
      return NULL;
    }
    /* or if one subtree is null */
    else if(root->left == NULL) {
      p = root->right;
      free(root);
      return p;
    }
    else if(root->right == NULL) {
      p = root->left;
      free(root);
      return p;
    }
    /* or both subtrees present */
    else {
      p2 = root->right;
      p = root->right;
      while(p->left) p = p->left;
      p->left = root->left;
      free(root);
      return p2;
    }
  }
  if(root->info < key) root->right = dtree(root->right, key);
  else root->left = dtree(root->left, key);
  return root;
}