【数据结构之字典树Trie】C语言实现

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

/**
 * 字典树
 *  1、根节点（Root）不包含字符，除根节点外的每一个节点都仅包含一个字符；
 *  2、从根节点到某一节点路径上所经过的字符连接起来，即为该节点对应的字符串；
 *  3、任意节点的所有子节点所包含的字符都不相同；
 *  4、关键词的插入和查找过程的时间复杂度均为 O(key_length)，
 *  5、空间复杂度 O(ALPHABET_SIZE * key_length * N) ，其中 N 是关键词的数量。
**/
#define array_size(a) sizeof(a)/sizeof(a[0])

// Alphabet size (# of symbols)
#define ALPHABET_SIZE (26)

// Converts key current character into index
// use only 'a' through 'z' and lower case
#define char_to_index(c) ((int)c - (int)'a')

// trie node
typedef struct trie_node TrieNode;
struct trie_node
{
    int value;
    TrieNode* children[ALPHABET_SIZE];
};

// trie ADT
typedef struct trie TRIE;
struct trie
{
    TrieNode* root;
    int count;
};

// Returns new trie node (initialized to NULLs)
TrieNode *getNode(void)
{
    TrieNode *pNode = NULL;

    pNode = (TrieNode *)malloc(sizeof(TrieNode));

    if (pNode)
    {
        int i;
        pNode->value = 0;
        for (i = 0; i < ALPHABET_SIZE; i++)
        {
            pNode->children[i] = NULL;
        }
    }
    return pNode;
}

// Initializes trie (root is dummy node)
void initialize(TRIE *pTrie)
{
    pTrie->root = getNode();
    pTrie->count = 0;
}

// If not present, inserts key into trie
// If the key is prefix of trie node, just marks leaf node
void insert(TRIE *pTrie, char key[])
{
    int level;
    int length = strlen(key);
    int index;
    TrieNode *pCrawl;

    pTrie->count++;
    pCrawl = pTrie->root;

    for (level = 0; level < length; level++)
    {
        index = char_to_index(key[level]);
        if (!pCrawl->children[index])
        {
            pCrawl->children[index] = getNode();
        }

        pCrawl = pCrawl->children[index];
    }

    // mark last node as leaf
    pCrawl->value = pTrie->count;
}

// Returns non zero, if key presents in trie
int search(TRIE *pTrie, const char *key)
{
    int level;
    int length = strlen(key);
    int index;
    TrieNode *pCrawl;

    pCrawl = pTrie->root;

    for (level = 0; level < length; level++)
    {
        index = char_to_index(key[level]);

        if (!pCrawl->children[index])
        {
            return 0;
        }

        pCrawl = pCrawl->children[index];
    }

    return (0 != pCrawl && pCrawl->value);
    // return pCrawl->value;
}

// Driver
int main()
{
    // Input keys (use only 'a' through 'z' and lower case)
    char keys[][8] = { "the", "a", "there", "answer", "any", "by", "bye", "their" };

    char output[][32] = { "Not present in trie", "Present in trie" };

    TRIE trie;
    initialize(&trie);

    // Construct trie
    for (int i = 0; i < array_size(keys); i++)
    {
        insert(&trie, keys[i]);
    }

    // Search for different keys
    printf("%s --- %s\n", "the", output[search(&trie, "the")]);
    printf("%s --- %s\n", "these", output[search(&trie, "these")]);
    printf("%s --- %s\n", "their", output[search(&trie, "their")]);
    printf("%s --- %s\n", "thaw", output[search(&trie, "thaw")]);

    printf("%d\n",search(&trie, "the"));
    printf("%d\n",search(&trie, "these"));
    printf("%d\n",search(&trie, "their"));
    printf("%d\n",search(&trie, "thaw"));

    return 0;
}