added skip_list.cpp

data_structures/skip_list.cpp

@@ -0,0 +1,185 @@
+/**
+ * A skip list is a data structure that is used for storing a sorted list of items with a
+ *  help of hierarchy of linked lists that connect increasingly sparse subsequences of the items
+ * 
+ *  References used: GeeksForGeeks, https://iq.opengenus.org/skip-list/ Pseudo Code, https://ideone.com/XnXIFl from CodeForces.
+*/
+
+#include <iostream>
+#include <vector>
+#include <stdio.h>
+#include <string.h>
+using namespace std; 
+
+#define MAXLVL 2
+#define P 0.5
+  
+
+/**
+ *  Node structure [Key][Node*, Node*...]
+*/
+class Node { 
+public: 
+    int key; 
+    void* value;
+    /*Forward Array*/
+    vector<Node*> forward;
+    Node(int key, int level, void* value); 
+};
+
+  
+Node::Node(int key, int level, void* value) { 
+    this->key = key; 
+  
+    /*Initialization of forward vector*/
+    for (int i = 0; i < sizeof(Node*)*(level+1); i++){
+        forward.push_back(NULL);
+    }
+}; 
+  
+// Class for Skip list 
+class SkipList { 
+    int level; 
+    Node *header; 
+public: 
+    SkipList(); 
+    int randomLevel(); 
+    void insertElement(int, void*); 
+    void deleteElement(int); 
+    void* searchElement(int); 
+    void displayList(); 
+}; 
+  
+SkipList::SkipList() { 
+    level = 0; 
+    /* Header initialization*/
+    header = new Node(-1, MAXLVL, NULL); 
+}; 
+
+  
+/**
+ * Returns random level for skip list;
+*/
+int SkipList::randomLevel() { 
+    int lvl = 0; 
+    while(rand()%2 < P && lvl < MAXLVL) lvl++;  
+    return lvl; 
+}; 
+  
+  
+// Insert given key in skip list 
+void SkipList::insertElement(int key, void* value) { 
+    printf("Inserting %d key ... ", key);
+    Node *x = header; 
+    Node *update[MAXLVL+1]; 
+    memset(update, 0, sizeof(Node*)*(MAXLVL+1)); 
+
+
+    for(int i = level; i >= 0; i--) { 
+        while(x->forward[i] != NULL && x->forward[i]->key < key)  x = x->forward[i]; 
+        update[i] = x; 
+    } 
+
+    x = x->forward[0]; 
+
+    bool doesnt_exist = (x == NULL || x->key != key);
+    if (doesnt_exist) { 
+        int rlevel = randomLevel(); 
+  
+        if(rlevel > level) { 
+            for(int i=level+1;i<rlevel+1;i++) update[i] = header; 
+
+            /*Update current level*/
+            level = rlevel; 
+        } 
+  
+        Node* n = new Node(key, rlevel, value);
+        for(int i=0;i<=rlevel;i++) { 
+            n->forward[i] = update[i]->forward[i]; 
+            update[i]->forward[i] = n; 
+        } 
+        printf(" Inserted\n");
+    } else {
+        printf("Exists\n");
+    }
+}; 
+  
+// Delete element from skip list 
+void SkipList::deleteElement(int key) 
+{ 
+    Node *x = header; 
+  
+    Node *update[MAXLVL+1]; 
+    memset(update, 0, sizeof(Node*)*(MAXLVL+1)); 
+  
+    for(int i = level; i >= 0; i--)  { 
+        while(x->forward[i] != NULL  &&  x->forward[i]->key < key)  x = x->forward[i]; 
+        update[i] = x; 
+    } 
+  
+    x = x->forward[0]; 
+  
+    bool doesnt_exist = (x == NULL || x->key != key);
+
+    if(!doesnt_exist) { 
+        for(int i=0;i<=level;i++) {
+            if(update[i]->forward[i] != x)  break; 
+            update[i]->forward[i] = x->forward[i]; 
+        }
+       /*Remove empty levels*/
+        while(level>0 &&  header->forward[level] == 0) level--; 
+        printf("Deleted\n");
+    } else {
+        printf("Doesnt Exists\n");
+    }
+}; 
+  
+
+/**
+ * Searching element in skip list structure
+*/
+void* SkipList::searchElement(int key) { 
+    Node *x = header; 
+    printf("Searching for %d\n", key);
+
+    for(int i = level; i >= 0; i--) { 
+        while(x->forward[i] && x->forward[i]->key < key)  x = x->forward[i]; 
+    } 
+  
+    x = x->forward[0]; 
+    if(x && x->key == key){
+        printf("FOUND\n");
+        return x->value;
+    } else {
+        printf("NOT FOUND\n");
+        return NULL;
+    }
+}; 
+  
+// Display skip list level wise 
+void SkipList::displayList() { 
+    printf("----SKIP LIST STRUCTURE----\n");
+    for(int i=0; i <= level; i++) { 
+        Node *node = header->forward[i]; 
+        printf("Level %d: ", i);
+        while(node != NULL) { 
+            printf("%d ", node->key);
+            node = node->forward[i]; 
+        } 
+        printf("\n");
+    } 
+}; 
+  
+int main() 
+{ 
+    srand((unsigned)time(0)); 
+  
+    SkipList lst;
+
+    for (int j = 0; j < (1 << (MAXLVL+1)); j++){
+         int k = (rand() % (1 << (MAXLVL+1)  + 1));
+         lst.insertElement(k, &j); 
+    }
+
+    lst.displayList();
+}