# Hashtables Chaining with Doubly Linked Lists

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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