Trie 树
最近在看国外某个开源的 C语言算法库，可能知名度不是很高，但是比较容易看懂，适合入门，今天看到了一个Trie这个文件的代码，原本不知道这是什么意思的，看来一下代码之后发现每个节点都会有256大小的节点数组，256和什么对应呢？没错，作为一个程序员你应该对256这个数字很熟悉，那是因为有256个ASSIC，这里256是一个byte的大小。实际上也就是将字符转换为字节形式（无 符号非负整数），然后从高字节向低字节逐个检测，按照字节所表示的数字放到对应数组的位置上，在这个位置建立一个结点node，再取出下一个字节，对应放 入node的数组位置上，直到字符串的字节形式被完全遍历，那么就得到了这个字符串所在的结点。
下面是wiki百科对trie树的简单解释，附有例子代码
http://zh.wikipedia.org/wiki/Trie
下面是我看的库代码：
/*
Copyright (c) 2005-2008, Simon Howard
Permission to use, copy, modify, and/or distribute this software
for any purpose with or without fee is hereby granted, provided
that the above copyright notice and this permission notice appear
in all copies.
THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR
CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/* Trie: fast mapping of strings to values */
#include <stdlib.h>
#include <string.h>
#include "trie.h"
/* malloc() / free() testing */
#ifdef ALLOC_TESTING
#include "alloc-testing.h"
#endif
typedef struct _TrieNode TrieNode;
struct _TrieNode {
TrieValue data;
unsigned int use_count;
TrieNode *next[256];
};
struct _Trie {
TrieNode *root_node;
};
Trie *trie_new(void)
{
Trie *new_trie;
new_trie = (Trie *) malloc(sizeof(Trie));
if (new_trie == NULL) {
return NULL;
}
new_trie->root_node = NULL;
return new_trie;
}
static void trie_free_list_push(TrieNode **list, TrieNode *node)
{
node->data = *list;
*list = node;
}
static TrieNode *trie_free_list_pop(TrieNode **list)
{
TrieNode *result;
result = *list;
*list = result->data;
return result;
}
void trie_free(Trie *trie)
{
TrieNode *free_list;
TrieNode *node;
int i;
free_list = NULL;
/* Start with the root node */
if (trie->root_node != NULL) {
trie_free_list_push(&free_list, trie->root_node);
}
/* Go through the free list, freeing nodes. We add new nodes as
* we encounter them; in this way, all the nodes are freed
* non-recursively. */
while (free_list != NULL) {
node = trie_free_list_pop(&free_list);
/* Add all children of this node to the free list */
for (i=0; i<256; ++i) {
if (node->next[i] != NULL) {
trie_free_list_push(&free_list, node->next[i]);
}
}
/* Free the node */
free(node);
}
/* Free the trie */
free(trie);
}
static TrieNode *trie_find_end(Trie *trie, char *key)
{
TrieNode *node;
char *p;
int c;
/* Search down the trie until the end of string is reached */
node = trie->root_node;
for (p=key; *p != '\0'; ++p) {
if (node == NULL) {
/* Not found in the tree. Return. */
return NULL;
}
/* Jump to the next node */
c = *p;
node = node->next[c];
}
/* This key is present if the value at the last node is not NULL */
return node;
}
/* Roll back an insert operation after a failed malloc() call. */
static void trie_insert_rollback(Trie *trie, char *key)
{
TrieNode *node;
TrieNode **prev_ptr;
TrieNode *next_node;
TrieNode **next_prev_ptr;
char *p;
/* Follow the chain along. We know that we will never reach the
* end of the string because trie_insert never got that far. As a
* result, it is not necessary to check for the end of string
* delimiter (NUL) */
node = trie->root_node;
prev_ptr = &trie->root_node;
p = key;
while (node != NULL) {
/* Find the next node now. We might free this node. */
next_prev_ptr = &node->next[(int) *p];
next_node = *next_prev_ptr;
++p;
/* Decrease the use count and free the node if it
* reaches zero. */
--node->use_count;
if (node->use_count == 0) {
free(node);
if (prev_ptr != NULL) {
*prev_ptr = NULL;
}
next_prev_ptr = NULL;
}
/* Update pointers */
node = next_node;
prev_ptr = next_prev_ptr;
}
}
int trie_insert(Trie *trie, char *key, TrieValue value)
{
TrieNode **rover;
TrieNode *node;
char *p;
int c;
/* Cannot insert NULL values */
if (value == TRIE_NULL) {
return 0;
}
/* Search to see if this is already in the tree */
node = trie_find_end(trie, key);
/* Already in the tree? If so, replace the existing value and
* return success. */
if (node != NULL && node->data != TRIE_NULL) {
node->data = value;
return 1;
}
/* Search down the trie until we reach the end of string,
* creating nodes as necessary */
rover = &trie->root_node;
p = key;
for (;;) {
node = *rover;
if (node == NULL) {
/* Node does not exist, so create it */
node = (TrieNode *) calloc(1, sizeof(TrieNode));
if (node == NULL) {
/* Allocation failed. Go back and undo
* what we have done so far. */
trie_insert_rollback(trie, key);
return 0;
}
node->data = TRIE_NULL;
/* Link in to the trie */
*rover = node;
}
/* Increase the node use count */
++node->use_count;
/* Current character */
c = *p;
/* Reached the end of string? If so, we're finished. */
if (c == '\0') {
/* Set the data at the node we have reached */
node->data = value;
break;
}
/* Advance to the next node in the chain */
rover = &node->next[c];
++p;
}
return 1;
}
int trie_remove(Trie *trie, char *key)
{
TrieNode *node;
TrieNode *next;
TrieNode **last_next_ptr;
char *p;
int c;
/* Find the end node and remove the value */
node = trie_find_end(trie, key);
if (node != NULL && node->data != TRIE_NULL) {
node->data = TRIE_NULL;
} else {
return 0;
}
/* Now traverse the tree again as before, decrementing the use
* count of each node. Free back nodes as necessary. */
node = trie->root_node;
last_next_ptr = &trie->root_node;
p = key;
for (;;) {
/* Find the next node */
c = *p;
next = node->next[c];
/* Free this node if necessary */
--node->use_count;
if (node->use_count <= 0) {
free(node);
/* Set the "next" pointer on the previous node to NULL,
* to unlink the freed node from the tree. This only
* needs to be done once in a remove. After the first
* unlink, all further nodes are also going to be
* free'd. */
if (last_next_ptr != NULL) {
*last_next_ptr = NULL;
last_next_ptr = NULL;
}
}
/* Go to the next character or finish */
if (c == '\0') {
break;
} else {
++p;
}
/* If necessary, save the location of the "next" pointer
* so that it may be set to NULL on the next iteration if
* the next node visited is freed. */
if (last_next_ptr != NULL) {
last_next_ptr = &node->next[c];
}
/* Jump to the next node */
node = next;
}
/* Removed successfully */
return 1;
}
TrieValue trie_lookup(Trie *trie, char *key)
{
TrieNode *node;
node = trie_find_end(trie, key);
if (node != NULL) {
return node->data;
} else {
return TRIE_NULL;
}
}
int trie_num_entries(Trie *trie)
{
/* To find the number of entries, simply look at the use count
* of the root node. */
if (trie->root_node == NULL) {
return 0;
} else {
return trie->root_node->use_count;
}
}
