《算法导论》第11章 散列表 (3)开放寻址

前一节介绍是最简单的冲突解决方法-链接法。开放寻址与链接法不同，所有元素都放在散列表内。

在这种方法中，散列表可能会被填满。开放寻址不需要指针，只需要计算出要存取的各个槽。

由于不用存储指针而节省的空间可以提供更多的槽。

有三种技术常用来计算开放寻址法中的探查序列：线性探查、二次探查和双重探查。

下面的实现中，三种方法的差别只在计算探查序列的那一行代码。

#include <stdio.h> #include <stdlib.h> #include <string.h> #define SIZE 20 typedef struct _Entry { char *key; char *val; } Entry; // 指针数组 Entry *hashmap[SIZE]; // Same as hashcode when String put to HashMap 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); } Entry * hashmap_search(char *key) { unsigned h = hashcode(key) % SIZE; unsigned h2 = h; Entry *entry = hashmap[h]; while (entry != NULL) { if (strcmp(entry->key, key) == 0) { return entry; } // 线性探查。不同探查方法差别只在这一行。 h = (h + 1) % SIZE; entry = hashmap[h]; if (h == h2) break; } 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 unsigned h2 = h; Entry *entry = hashmap[h]; while (entry != NULL) { if (strcmp(entry->key, key) == 0) { char *oldVal = entry->val; entry->val = val; return oldVal; } // Linear search h = (h + 1) % SIZE; entry = hashmap[h]; // Check if loop to initial bucket if (h == h2) break; } // Not found, create new node to save key&val pair if (entry == NULL) { entry = malloc(sizeof(Entry)); entry->key = key; entry->val = val; hashmap[h] = entry; } return val; } void hashmap_print() { Entry *entry; int i; for (i = 0; i < SIZE; i++) { entry = hashmap[i]; if (entry == NULL) printf("%d: null\n", i); else printf("%d: %s - %s\n", i, entry->key, entry->val); } 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); hashmap_print(); return 1; }