Merge pull request #180 from Deepak25101997/master

Selection Sort in Singly Linked List

Author: ashwek

Operations on Datastructures/selectionSortLinkedList.cpp

@@ -0,0 +1,162 @@
+#include <iostream>
+using namespace std;
+
+
+//node defined
+class node
+{
+public:
+    int data;
+    node* link;
+    node(int d)
+    {
+        data = d;
+        link = NULL;
+    }
+
+};
+
+//printing the linked list
+void print(node* head)
+{
+    node* current = head;
+    while (current != NULL)
+    {
+        cout << current->data << " ";
+        current = current-> link;
+    }
+    cout << endl;
+}
+
+//creating the linked list with 'n' nodes
+node* createlist(int n)
+{
+    node* head = NULL;
+    node* t = NULL;
+    for (int i = 0; i < n; i++)
+    {
+        node* temp = NULL;
+        int num;
+        cin >> num;
+        temp = new node(num);
+        if (head == NULL)
+        {
+            head = temp;
+            t = temp;
+            continue;
+        }
+        if (t->link == NULL) t->link = temp;
+        t = temp;
+    }
+    return head;
+}
+
+
+//performing selection sort on the linked list in an iterative manner
+void my_selection_sort_linked_list(node* &head)
+{
+    node* min = head;			//throughout the algorithm 'min' is used to denote the node with min value out of all the nodes left for scanning
+					//while scanning if we find a node 'X' with value lesser than min, 
+					//then we update the pointers in such a way that 'X' becomes the predecessor of 'min'
+    node* current = min->link;		// 'current' refers to the current node we are scanning
+    node* previous = min;		//'previous' refers to the node that is previous to the current node
+    node* temp = NULL;			// 'temp' in this algo is used to point to the last node of the sorted part of the linked list.
+					//eg. If at any time instance the state of the linked list is suppose 1->2->5->3->8->NULL
+					//then, we see that "1->2" is the sorted part of the LL, and therefore temp will be pointing to the last node of the sorted part,i.e,'2'
+	//We keep on arranging the Linked list in such a way that after each iteration the node with 'min' value is placed at its correct position.
+	//Eg. Let suppose initially we have 5->4->1->3->2->NULL
+	//After 1st iteration : 1->4->5->3->2->NULL and so on  
+ 
+   while (min->link != NULL)			//so that all the nodes are scanned or until there exists a node
+    {
+						//pick the first node from the unsorted part and assume that it is the minimum and then start scanning from the next node
+
+        while (current != NULL)			//suppose you choose the min node to be X, then scan starts from the (X+1)th node until its NULL. current = (X+1)th node and min = X
+        {
+            if (current->data < min->data)	//if the current node is smaller than the presumed node 'min' 
+            {
+                if (temp == NULL)		//temp stays null for the first iteration, therefore it symbolizes that we are scanning for the first time
+                {
+                    if (previous == min)	//if the 'previous' is pointing to the 'min' node
+                    {
+			//Update the pointers
+                        head = current;		//update the head pointer with the current node
+                        min->link = current->link;
+                        current->link = previous;
+                        min = current;
+                        current = previous->link;
+                    }
+                    else				//if the 'previous' is not pointing to the 'min' node
+                    {
+			//Update the pointers
+                        head = current;			//update the head pointer with the current node
+                        previous->link = current->link;
+                        current->link = min;
+                        min = current;
+                        current = previous->link;
+                    }
+                }
+                else					//if 'temp' is not NULL, i.e., its not the 1st iteration
+                {
+                    temp->link = current;
+                    previous->link = current->link;
+                    current->link = min;
+                    min = current;
+                    current = previous->link;
+                }
+            }
+            else				//if the current node is greater than min, just move the previous and the current pointer a step further
+            {
+                previous = previous->link;
+                current = current->link;
+            }
+        }
+
+	//update the pointers. Set 'temp' to the last node in the sorted part. Make 'min' move a step further so that 'min' points to the 1st node of the unsorted part
+	//start the iteration again
+        temp = min;
+        min = min->link;
+        previous = min;
+        current = min->link;
+    }
+}
+
+// Test cases:
+
+// enter the no. of nodes : 5
+// 8 9 3 1 4
+// original list is : 8 9 3 1 4
+// sorted list is : 1 3 4 8 9
+
+// enter the no. of nodes : 3
+// -1 -2 -3
+// original list is : -1 -2 -3
+// sorted list is : -3 -2 -1
+
+
+// enter the no. of nodes : 8
+// 8 7 6 5 4 3 2 1
+// original list is : 8 7 6 5 4 3 2 1
+// sorted list is : 1 2 3 4 5 6 7 8
+
+// enter the no. of nodes : 6
+// 5 3 4 1 -2 -4
+// original list is : 5 3 4 1 -2 -4
+// sorted list is : -4 -2 1 3 4 5
+
+
+int main()
+{
+    node* head = NULL;
+    int n;
+    cout << "enter the no. of nodes : ";    //taking input from user about the number of nodes in linked list
+    cin >> n;
+    if (n == 0) return 0;
+    head = createlist(n);                   //creating the list
+    cout << "original list is : ";
+    print(head);                            //printing the original linked list
+    my_selection_sort_linked_list(head);    //applying selection sort
+    cout << "sorted list is : ";
+    print(head);                            //printing the sorted linked list
+    return 0;
+}