一、哈希表概述

1、散列表（Hash table，也叫哈希表），是根据关键码值(Key value)而直接进行访问的数据结构。它通过把关键码值映射到表中一个位置来访问记录，以加快查找的速度。这个映射函数叫做散列函数，存放记录的数组叫做散列表。

2、散列函数：用于将键映射到特定的桶，理想的散列函数，将键均匀分布在哈希桶中，最大程度地减少散列冲突（哈希碰撞）。散列函数，常用取余法，即将key对base取余，对应的余数为哈希桶的索引；base通常取质数，可以尽量保证key均匀分布在哈希桶中；哈希桶的数量为base。

3、散列冲突：不同的键通过哈希函数映射到同一个哈希桶中。解决散列冲突，常用拉链法和开放定址法（线性探测法）。拉链法，是指产生hash冲突后在存储数据后面加一个指针，指向后面冲突的数据，如下图：

二、手动设计哈希集合与哈希映射

leetcode_705. 设计哈希集合哈希集合：

struct List {
    int val;
    struct List *next;
};

void listPush(struct List *head, int key)
{
    // 头插法
    struct List *temp = (struct List *)malloc(sizeof(struct List));
    temp->val = key;
    temp->next = head->next;
    head->next = temp;
}

void listDelete(struct List *head, int key)
{
    struct List *temp = head;
    while (temp->next != NULL) {
        if (temp->next->val == key) {
            struct List *node = temp->next;
            temp->next = node->next;
            free(node);
            return;
        }
        temp = temp->next;
    }
}

bool listContain(struct List *head, int key)
{
    struct List *temp = head->next;
    while (temp != NULL) {
        if (temp->val == key) {
            return true;
        }
        temp = temp->next;
    }
    return false;
}

void listFree(struct List *head)
{
    struct List *temp = NULL;
    while (head->next != NULL) {
        temp = head->next;
        head->next = temp->next;
        free(temp);
    }
}

const int base = 769;

int hash(int key) {
    return key % base;
}

typedef struct {
    struct List *data;
} MyHashSet;

/** Initialize your data structure here. */

MyHashSet* myHashSetCreate() {
    // 拉链法
    MyHashSet *obj = (MyHashSet*)malloc(sizeof(MyHashSet));
    obj->data = (struct List*)malloc(sizeof(struct List) * base);
    for (int i = 0; i < base; i++) {
        // 头节点不参加判断，赋任意值，不影响
        obj->data[i].val = 0;
        obj->data[i].next = NULL;
    }
    return obj;
}

void myHashSetAdd(MyHashSet* obj, int key) {
    int h = hash(key);
    if (listContain(&(obj->data[h]), key) != true) {
        listPush(&(obj->data[h]), key);
    }
}

void myHashSetRemove(MyHashSet* obj, int key) {
    int h = hash(key);
    listDelete(&(obj->data[h]), key);
}

/** Returns true if this set contains the specified element */
bool myHashSetContains(MyHashSet* obj, int key) {
    int h = hash(key);
    return listContain(&(obj->data[h]), key);
}

void myHashSetFree(MyHashSet* obj) {
    for (int i = 0; i < base; i++) {
        listFree(&(obj->data[i]));
    }
    free(obj->data);
}

/**
 * Your MyHashSet struct will be instantiated and called as such:
 * MyHashSet* obj = myHashSetCreate();
 * myHashSetAdd(obj, key);
 
 * myHashSetRemove(obj, key);
 
 * bool param_3 = myHashSetContains(obj, key);
 
 * myHashSetFree(obj);
*/

leetcode_706. 设计哈希映射

struct List {
    int key;
    int val;
    struct List *next;
};

void hashMapPut(struct List *head, int key, int val)
{
    // 头插法
    struct List *temp = head->next;
    while (temp != NULL) {
        if (temp->key == key) {
            return temp->val = val;
        }
        temp = temp->next;
    }
    
    struct List *node = (struct List *)malloc(sizeof(struct List));
    node->key = key;
    node->val = val;
    node->next = head->next;

    head->next = node;
}

int hashMapGet(struct List *head, int key)
{
    struct List *temp = head->next;
    while (temp != NULL) {
        if (temp->key == key) {
            return temp->val;
        }
        temp = temp->next;
    }
    return -1;
}

void hashMapRemove(struct List *head, int key)
{
    struct List *temp = NULL;
    while (head->next != NULL) {
        if (head->next->key == key) {
            temp = head->next;
            head->next = temp->next;
            free(temp);
            return;
        }
        head = head->next;
    }
}

void hashMapFree(struct List *head)
{
    struct List *temp = NULL;
    while (head->next != NULL) {
        temp = head->next;
        head->next = temp->next;
        free(temp);
    }
}
const int base = 769;
int hash(int key)
{
    return key % 769;
}


typedef struct {
    struct List *data;
} MyHashMap;

/** Initialize your data structure here. */

MyHashMap* myHashMapCreate() {
    MyHashMap *obj = (MyHashMap*)malloc(sizeof(MyHashMap));
    obj->data = (struct List*)malloc(base * sizeof(struct List));
    for (int i = 0; i < base; i++) {
        obj->data[i].key = 0;
        obj->data[i].val = 0;
        obj->data[i].next = NULL;
    }
    return obj;
}

/** value will always be non-negative. */
void myHashMapPut(MyHashMap* obj, int key, int value) {
    int h = hash(key);
    hashMapPut(&(obj->data[h]), key, value);
}

/** Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key */
int myHashMapGet(MyHashMap* obj, int key) {
    int h = hash(key);
    return hashMapGet(&(obj->data[h]), key);
}

/** Removes the mapping of the specified value key if this map contains a mapping for the key */
void myHashMapRemove(MyHashMap* obj, int key) {
    int h = hash(key);
    hashMapRemove(&(obj->data[h]), key);
}

void myHashMapFree(MyHashMap* obj) {
    for (int i = 0; i < base; i++) {
        hashMapFree(&(obj->data[i]));
    }
    free(obj->data);
}

/**
 * Your MyHashMap struct will be instantiated and called as such:
 * MyHashMap* obj = myHashMapCreate();
 * myHashMapPut(obj, key, value);
 
 * int param_2 = myHashMapGet(obj, key);
 
 * myHashMapRemove(obj, key);
 
 * myHashMapFree(obj);
*/

三、ustash使用

1、概述
（1）ustash是C语言中哈希函数集合，本质上是一组宏定义；使用时需要包含头文件uthash.h;

（2）常用结构体：

typedef struct {
    int id;
	int val;
    UT_hash_handle hh;    // 必须定义
} HASH_TABLE;

（3）常用宏定义：
参考： https://blog.csdn.net/fan_h_l/article/details/107241520

增加元素：HASH_ADD_INT

查找元素：HASH_FIND_INT

删除元素：HASH_DEL

遍历元素：HASH_ITER

统计元素：HASH_COUNT

2、leetcode_349. 两个数组的交集

/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
typedef struct {
    int id;
    UT_hash_handle hh;
} HASH_TABLE;

int* intersection(int* nums1, int nums1Size, int* nums2, int nums2Size, int* returnSize)
{
    int i;
    int count = 0;
    // 指针必须初始化
    HASH_TABLE *table1 = NULL;
    HASH_TABLE *table2 = NULL;
    HASH_TABLE *temp= NULL;
    *returnSize = 0;

    for(i = 0; i < nums1Size; i++) {
        HASH_FIND_INT(table1, &(nums1[i]), temp);
        if (temp != NULL) {
            continue;
        }
        temp = (HASH_TABLE*)malloc(sizeof(HASH_TABLE));
        temp->id = nums1[i];
        HASH_ADD_INT(table1, id, temp);
    }

    for(i = 0; i < nums2Size; i++) {
        HASH_FIND_INT(table2, &(nums2[i]), temp);
        if (temp != NULL) {
            continue;
        }
        HASH_FIND_INT(table1, &(nums2[i]), temp);
        if (temp == NULL) {
            continue;
        }

        temp = (HASH_TABLE*)malloc(sizeof(HASH_TABLE));
        temp->id = nums2[i];
        HASH_ADD_INT(table2, id, temp);
        count++;     
    }

    int *ret = (int*)malloc(count * sizeof(int));
    for (temp = table2, i = 0; temp != NULL; temp = (HASH_TABLE*)temp->hh.next) {
        ret[i++] = temp->id;
    }
    
    *returnSize = count;
    return ret;
}

数据结构之哈希表