GH-147: Skiplist in C++

README.md

@@ -179,6 +179,23 @@ A data structure is a data organization, management, and storage format that is
 </tbody>
 </table>
 
+**Advanced Data Structures and Algorithms**
+
+<table>
+<thead>
+<th>Advanced Concept</th>
+<th>Programming Languages</th>
+</thead>
+<tbody>
+<tr>
+<td>Skiplist</td>
+<td>
+<a href="/concepts/cpp/skip_list.md"><code>cpp🐀</code></a>
+</td>
+</tr>
+</tbody>
+</table>
+
 ## 🔆 Collections
 
 **Competitive Programming Events**

concepts/cpp/skip_list.md

@@ -0,0 +1,211 @@
+# Skip list
+
+From wikipedia
+
+> A skip list (or skiplist) is a probabilistic data structure that allows $O(log\ n)$
+average complexity for search as well as $O(log\ n)$ average complexity for insertion 
+within an ordered sequence of n elements.
+
+![](../abstract/data-structures/skip_list.png)
+
+## Time complexity of Skip list
+
+|            | Time complexity         |
+|------------|-------------------------|
+| Insertion  | $O(log\ n)$              |
+| Deletion   | $O(log\ n)$              |
+| Search     | $O(log\ n)$              |
+| Access     | $O(n)$                  |
+
+## 💻 Implementation
+
+ℹ️ Here is the implementation of the Skip list in C++
+
+```cpp
+#include <climits>
+#include <iostream>
+
+using namespace std;
+
+struct Node {
+  int key;
+  int level;
+  Node** forwards;
+  // constructor
+  Node(int key, int level) {
+    this->key = key;
+    this->level = level;
+    forwards = new Node*[level + 1];
+    for (int i = 0; i <= level; i++) {
+      forwards[i] = NULL;
+    }
+  }
+};
+
+class Skiplist {
+ public:
+  Skiplist(int max_level);
+  ~Skiplist();
+  void insert(int key);
+  bool search(int key);
+  void remove(int key);
+  void show();
+
+ private:
+  Node* head;
+  int MAX_LEVEL;
+};
+
+Skiplist::Skiplist(int max_level = 4): MAX_LEVEL(max_level) {
+  head = new Node(INT_MIN, MAX_LEVEL);
+  srand(time(NULL));
+}
+
+Skiplist::~Skiplist() { delete head; }
+
+void Skiplist::insert(int key) {
+  int level = 0;
+  while (rand() % 2 && level < MAX_LEVEL) {
+    level++;
+  }
+  Node* newNode = new Node(key, level);
+
+  Node* current = head;
+  Node** update = new Node*[level + 1];
+  for (int i = 0; i <= level; i++) {
+    update[i] = NULL;
+  }
+
+  for (int i = level; i >= 0; i--) {
+    while (current->forwards[i] != NULL && current->forwards[i]->key < key) {
+      current = current->forwards[i];
+    }
+    update[i] = current;
+  }
+
+  for (int i = 0; i <= level; i++) {
+    newNode->forwards[i] = update[i]->forwards[i];
+    update[i]->forwards[i] = newNode;
+  }
+}
+
+bool Skiplist::search(int key) {
+  Node* current = head;
+  for (int i = MAX_LEVEL; i >= 0; i--) {
+    while (current->forwards[i] != NULL && current->forwards[i]->key < key) {
+      current = current->forwards[i];
+    }
+  }
+  if (current->forwards[0] != NULL && current->forwards[0]->key == key) {
+    return true;
+  }
+  return false;
+}
+
+void Skiplist::remove(int key) {
+  Node* current = head;
+  Node** update = new Node*[MAX_LEVEL + 1];
+  for (int i = 0; i <= MAX_LEVEL; i++) {
+    update[i] = NULL;
+  }
+
+  for (int i = MAX_LEVEL; i >= 0; i--) {
+    while (current->forwards[i] != NULL && current->forwards[i]->key < key) {
+      current = current->forwards[i];
+    }
+    update[i] = current;
+  }
+
+  current = current->forwards[0];
+
+  if (current && current->key == key) {
+    for (int i = 0; i <= MAX_LEVEL; i++) {
+      if (update[i]->forwards[i] != current) {
+        break;
+      }
+      update[i]->forwards[i] = current->forwards[i];
+    }
+    delete current;
+  }
+  delete[] update;
+}
+
+void Skiplist::show() {
+  Node* current = head;
+  while (current->forwards[0] != NULL) {
+    cout << current->forwards[0]->key << " ";
+    current = current->forwards[0];
+  }
+  cout << endl;
+
+  for (int i = MAX_LEVEL; i >= 0; i--) {
+    cout << "Level " << i << ": ";
+    Node* current = head;
+    while (current->forwards[i] != NULL) {
+      cout << current->forwards[i]->key << " -> ";
+      current = current->forwards[i];
+    }
+    cout << endl;
+  }
+}
+
+int main(){
+  cout << "Skiplist Program" << endl;
+
+  // create an skiplist
+  Skiplist sl;
+
+  cout << "\nInsert elements into Skiplist" << endl;
+  sl.insert(4);
+  sl.insert(2);
+  sl.insert(3);
+  sl.insert(9);
+  sl.insert(1);
+  sl.show();
+
+  cout << "\nSearch elements" << endl;
+  cout << "- Search for 3: " << sl.search(3) << endl;
+  cout << "- Search for 5: " << sl.search(5) << endl;
+
+  cout << "\nRemove elements" << endl;
+  sl.remove(3);
+  sl.remove(5);  
+  sl.show();
+
+  return 0;
+}
+```
+
+Output
+
+```
+Skiplist Program
+
+Insert elements into Skiplist
+1 2 3 4 9 
+Level 4: 
+Level 3: 1 -> 
+Level 2: 1 -> 
+Level 1: 1 -> 9 -> 
+Level 0: 1 -> 2 -> 3 -> 4 -> 9 -> 
+
+Search elements
+- Search for 3: 1
+- Search for 5: 0
+
+Remove elements
+1 2 4 9 
+Level 4: 
+Level 3: 1 -> 
+Level 2: 1 -> 
+Level 1: 1 -> 9 -> 
+Level 0: 1 -> 2 -> 4 -> 9 -> 
+```
+
+## 🔗 Further Reading
+
+* ▶️ [Skip Lists](https://www.youtube.com/watch?v=NDGpsfwAaqo&ab_channel=AlgorithmsLab), Algorithms Lab
+* [Skip list](https://en.wikipedia.org/wiki/Skip_list), wikipedia
+* ▶️ [8 Key Data Structures That Power Modern Databases](https://www.youtube.com/watch?v=W_v05d_2RTo&ab_channel=ByteByteGo)
+
+

problems/skiplist/README.md

@@ -0,0 +1,7 @@
+# Usage
+# Skiplist 
+
+
+```
+bazel run src:skiplist
+```

problems/skiplist/src/BUILD

@@ -0,0 +1,8 @@
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+
+cc_binary(
+    name = "skiplist",
+    srcs = ["skiplist.cpp", "skiplist.h"],
+    copts = ["-std=c++17", "-Wall", "-Wextra", "-Werror"],
+    deps = [],
+)

problems/skiplist/src/skiplist.cpp

@@ -0,0 +1,30 @@
+#include <iostream>
+#include "skiplist.h"
+
+using namespace std;
+
+int main(){
+  cout << "Skiplist Program" << endl;
+
+  // create an skiplist
+  Skiplist sl;
+
+  cout << "\nInsert elements into Skiplist" << endl;
+  sl.insert(4);
+  sl.insert(2);
+  sl.insert(3);
+  sl.insert(9);
+  sl.insert(1);
+  sl.show();
+
+  cout << "\nSearch elements" << endl;
+  cout << "- Search for 3: " << sl.search(3) << endl;
+  cout << "- Search for 5: " << sl.search(5) << endl;
+
+  cout << "\nRemove elements" << endl;
+  sl.remove(3);
+  sl.remove(5);  
+  sl.show();
+
+  return 0;
+}