Create merge_sort

merge_sort

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+// C++ code for linked list merged sort 
+#include <bits/stdc++.h> 
+using namespace std; 
+
+/* Link list node */
+class Node { 
+public: 
+	int data; 
+	Node* next; 
+}; 
+
+/* function prototypes */
+Node* SortedMerge(Node* a, Node* b); 
+void FrontBackSplit(Node* source, 
+					Node** frontRef, Node** backRef); 
+
+/* sorts the linked list by changing next pointers (not data) */
+void MergeSort(Node** headRef) 
+{ 
+	Node* head = *headRef; 
+	Node* a; 
+	Node* b; 
+
+	/* Base case -- length 0 or 1 */
+	if ((head == NULL) || (head->next == NULL)) { 
+		return; 
+	} 
+
+	/* Split head into 'a' and 'b' sublists */
+	FrontBackSplit(head, &a, &b); 
+
+	/* Recursively sort the sublists */
+	MergeSort(&a); 
+	MergeSort(&b); 
+
+	/* answer = merge the two sorted lists together */
+	*headRef = SortedMerge(a, b); 
+} 
+
+/* See https:// www.geeksforgeeks.org/?p=3622 for details of this 
+function */
+Node* SortedMerge(Node* a, Node* b) 
+{ 
+	Node* result = NULL; 
+
+	/* Base cases */
+	if (a == NULL) 
+		return (b); 
+	else if (b == NULL) 
+		return (a); 
+
+	/* Pick either a or b, and recur */
+	if (a->data <= b->data) { 
+		result = a; 
+		result->next = SortedMerge(a->next, b); 
+	} 
+	else { 
+		result = b; 
+		result->next = SortedMerge(a, b->next); 
+	} 
+	return (result); 
+} 
+
+/* UTILITY FUNCTIONS */
+/* Split the nodes of the given list into front and back halves, 
+	and return the two lists using the reference parameters. 
+	If the length is odd, the extra node should go in the front list. 
+	Uses the fast/slow pointer strategy. */
+void FrontBackSplit(Node* source, 
+					Node** frontRef, Node** backRef) 
+{ 
+	Node* fast; 
+	Node* slow; 
+	slow = source; 
+	fast = source->next; 
+
+	/* Advance 'fast' two nodes, and advance 'slow' one node */
+	while (fast != NULL) { 
+		fast = fast->next; 
+		if (fast != NULL) { 
+			slow = slow->next; 
+			fast = fast->next; 
+		} 
+	} 
+
+	/* 'slow' is before the midpoint in the list, so split it in two 
+	at that point. */
+	*frontRef = source; 
+	*backRef = slow->next; 
+	slow->next = NULL; 
+} 
+
+/* Function to print nodes in a given linked list */
+void printList(Node* node) 
+{ 
+	while (node != NULL) { 
+		cout << node->data << " "; 
+		node = node->next; 
+	} 
+} 
+
+/* Function to insert a node at the beginging of the linked list */
+void push(Node** head_ref, int new_data) 
+{ 
+	/* allocate node */
+	Node* new_node = new Node(); 
+
+	/* put in the data */
+	new_node->data = new_data; 
+
+	/* link the old list off the new node */
+	new_node->next = (*head_ref); 
+
+	/* move the head to point to the new node */
+	(*head_ref) = new_node; 
+} 
+
+/* Drier program to test above functions*/
+int main() 
+{ 
+	/* Start with the empty list */
+	Node* res = NULL; 
+	Node* a = NULL; 
+
+	/* Let us create a unsorted linked lists to test the functions 
+Created lists shall be a: 2->3->20->5->10->15 */
+	push(&a, 15); 
+	push(&a, 10); 
+	push(&a, 5); 
+	push(&a, 20); 
+	push(&a, 3); 
+	push(&a, 2); 
+
+	/* Sort the above created Linked List */
+	MergeSort(&a); 
+
+	cout << "Sorted Linked List is: \n"; 
+	printList(a); 
+
+	return 0; 
+}