《算法导论》第11章 散列表 (2)散列表

用散列表来解决直接寻址表的那两个问题。但由此带来的散列值的碰撞问题。

最简单的解决方法是链接法，以及下一节介绍的开放寻址法。

链接法，即把散列到同一槽中的所有元素都放在一个链表中。

链表是无序的，在查找一个元素时需要遍历链表。

对于删除函数，假如参数是要删除的结点，那么如果链表是双向的，删除操作可以O(1)内完成。

在下面的删除函数中，参数是关键字，这样更为方便。

#include <stdio.h> #include <stdlib.h> #include <string.h> #define SIZE 20 // 链表结点的定义 typedef struct _ListNode { struct _ListNode *prev, *next; char *key; char *val; } ListNode; // 定义全局的各个槽链表的指针数组 ListNode *hashmap[SIZE]; // 这里的散列函数与Java中String及HashMap中的散列相同 // 注意为了保证向右逻辑运算（用0而不是符号位补高位） // 要将h声明为无符号的 unsigned hashcode(char *key) { // Ensure >> is logical shift unsigned h = 0; // String.hashcode() do h = 31 * h + *key++; while (*key != '\0'); // HashMap.hash() h ^= (h >> 20) ^ (h >> 12); return h ^ (h >> 7) ^ (h >> 4); } ListNode * hashmap_search(char *key) { unsigned h = hashcode(key) % SIZE; ListNode *node = hashmap[h]; while (node != NULL) { // 当相同时，strcmp返回0 if (strcmp(node->key, key) == 0) { return node; } node = node->next; } return NULL; } char * hashmap_insert(char *key, char *val) { unsigned h = hashcode(key) % SIZE; printf("Insert %s - %s to bucket %d\n", key, val, h); // Find duplicate key, replace it then return old value ListNode *node = hashmap_search(key); if (node != NULL) { char *oldVal = node->val; node->val = val; return oldVal; } // Not found, create new node to save key&val pair ListNode *newNode = malloc(sizeof(ListNode)); newNode->prev = NULL; newNode->key = key; newNode->val = val; if (hashmap[h] == NULL) { hashmap[h] = newNode; } else { hashmap[h]->prev = newNode; newNode->next = hashmap[h]; hashmap[h] = newNode; } return val; } char * hashmap_delete(char *key) { ListNode *node = hashmap_search(key); if (node != NULL) { // Set prev node's next to node's next if (node->prev == NULL) { unsigned h = hashcode(key) % SIZE; hashmap[h] = node->next; } else { node->prev->next = node->next; } // Set next node's prev to node's prev if (node->next != NULL) node->next->prev = node->prev; return node->val; } return NULL; } void hashmap_print() { int i; for (i = 0; i < SIZE; i++) { ListNode *node = hashmap[i]; while (node != NULL) { printf("%s - %s, ", node->key, node->val); node = node->next; } if (hashmap[i] != NULL) printf("%d bucket\n", i); } printf("\n"); } int main(void) { // Compare to String.hashcode() in JDK printf("%d\n", hashcode("helloworld")); hashmap_insert("aabb", "value1"); hashmap_insert("ccdd", "value2"); hashmap_insert("i'mcdai", "value3"); int i; for (i = 0; i < 2 * SIZE + 5; i++) { char *key = calloc(sizeof(char), 10); char *val = calloc(sizeof(char), 10); sprintf(key, "%s%d", "aabbcc", i); sprintf(val, "%s%d", "val ", i); hashmap_insert(key, val); } // Insert duplicate key printf("%s\n", hashmap_insert("i'mcdai", "dupdup")); hashmap_print(); printf("%s\n", hashmap_search("i'mcdai")->val); printf("%s\n", hashmap_search("aabbcc18")->val); //printf("%s\n", hashmap_search("NOTFOUND")->val); // All in bucket 18: aabbcc10 => aabbcc0 => i'mcdai printf("%s\n", hashmap_delete("aabbcc10")); printf("%s\n", hashmap_delete("i'mcdai")); hashmap_print(); return 1; }