Hashtables Chaining with Doubly Linked Lists

• Difficulty Level : Hard
• Last Updated : 19 Jun, 2018

Implementing hash table using Chaining through Doubly Linked List is similar to implementing Hashtable using Singly Linked List. The only difference is that every node of Linked List has the address of both, the next and the previous node. This will speed up the process of adding and removing elements from the list, hence the time complexity will be reduced drastically.
Example:

If we have a Singly linked list:

1->2->3->4

If we are at 3 and there is a need to remove it, then 2 need to be linked with 4 and as from 3, 2 can’t be accessed as it is singly linked list. So, the list has to be traversed again i.e O(n), but if we have doubly linked list i.e.

1<->2<->3<->4

2 & 4 can be accessed from 3, hence in O(1), 3 can be removed.

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Below is the implementation of the above approach:

 // C++ implementation of Hashtable // using doubly linked list #include using namespace std;    const int tablesize = 25;    // declaration of node struct hash_node {     int val, key;     hash_node* next;     hash_node* prev; };    // hashmap's declaration class HashMap { public:     hash_node **hashtable, **top;        // constructor     HashMap()     {         // create a empty hashtable         hashtable = new hash_node*[tablesize];         top = new hash_node*[tablesize];         for (int i = 0; i < tablesize; i++) {             hashtable[i] = NULL;             top[i] = NULL;         }     }        // destructor     ~HashMap()     {         delete[] hashtable;     }        // hash function definition     int HashFunc(int key)     {         return key % tablesize;     }        // searching method     void find(int key)     {         // Applying hashFunc to find         // index for given key         int hash_val = HashFunc(key);         bool flag = false;         hash_node* entry = hashtable[hash_val];            // if hashtable at that index has some         // values stored         if (entry != NULL) {             while (entry != NULL) {                 if (entry->key == key) {                     flag = true;                 }                 if (flag) {                     cout << "Element found at key "                          << key << ": ";                     cout << entry->val << endl;                 }                 entry = entry->next;             }         }         if (!flag)             cout << "No Element found at key "                  << key << endl;     }        // removing an element     void remove(int key)     {         // Applying hashFunc to find         // index for given key         int hash_val = HashFunc(key);         hash_node* entry = hashtable[hash_val];         if (entry->key != key || entry == NULL) {             cout << "Couldn't find any element at this key "                  << key << endl;             return;         }            // if some values are present at that key &         // traversing the list and removing all values         while (entry != NULL) {             if (entry->next == NULL) {                 if (entry->prev == NULL) {                     hashtable[hash_val] = NULL;                     top[hash_val] = NULL;                     delete entry;                     break;                 }                 else {                     top[hash_val] = entry->prev;                     top[hash_val]->next = NULL;                     delete entry;                     entry = top[hash_val];                 }             }             entry = entry->next;         }         cout << "Element was successfully removed at the key "              << key << endl;     }        // inserting method     void add(int key, int value)     {         // Applying hashFunc to find         // index for given key         int hash_val = HashFunc(key);         hash_node* entry = hashtable[hash_val];            // if key has no value stored         if (entry == NULL) {             // creating new node             entry = new hash_node;             entry->val = value;             entry->key = key;             entry->next = NULL;             entry->prev = NULL;             hashtable[hash_val] = entry;             top[hash_val] = entry;         }            // if some values are present         else {             // traversing till the end of             // the list             while (entry != NULL)                 entry = entry->next;                // creating the new node             entry = new hash_node;             entry->val = value;             entry->key = key;             entry->next = NULL;             entry->prev = top[hash_val];             top[hash_val]->next = entry;             top[hash_val] = entry;         }         cout << "Value " << value << " was successfully"                 " added at key " << key << endl;     } };    // Driver Code int main() {     HashMap hash;     hash.add(4, 5);     hash.find(4);     hash.remove(4);     return 0; }

Output:

Value 5 was successfully added at key 4
Element found at key 4: 5
Element was successfully removed at the key 4

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