Day-23 Linkedlist: reverse nodes and append

Author: mandliya

linked_list_problems/reverseAlternateNodes.cpp

@@ -0,0 +1,102 @@
+/*
+ * Given a linked list, reverse alternate nodes and append at the end
+ * Examples:
+ * Input List:  1->2->3->4->5->6
+ * Output List: 1->3->5->6->4->2
+ *
+ * Input List:  12->14->16->18->20
+ * Output List: 12->16->20->18->14
+ */
+
+#include <iostream>
+
+struct Node {
+  int data;
+  Node * next;
+  Node(int d) : data{ d }, next{ nullptr } { }
+};
+
+void insertAtHead( Node * & head, Node * node ) {
+  if ( head != nullptr ) {
+    node->next = head;
+  }
+  head = node;
+}
+
+void insert( Node * & head, int data ) {
+  Node * newNode = new Node(data);
+  if ( head == nullptr ) {
+    head = newNode;
+  } else {
+    Node * curr = head;
+    while( curr->next ) {
+      curr = curr->next;
+    }
+    curr->next = newNode;
+  }
+}
+
+void printList( Node * head ) {
+  while( head ) {
+    std::cout << head->data << "-->";
+    head = head->next;
+  }
+  std::cout << "NULL" << std::endl;
+ }
+
+ /* Algorithm function */
+ void rearrange( Node * & head ) {
+   if ( head == nullptr ) {
+     return;
+   }
+   Node * curr = head;
+   Node * forw = head->next;
+   if ( forw == nullptr ) {
+     return;
+   }
+   Node * backHead = nullptr;
+   while ( curr != nullptr && forw != nullptr ) {
+     curr->next = forw->next;
+     forw->next = nullptr;
+     insertAtHead( backHead, forw );
+     curr = curr->next;
+     if (curr != nullptr ) {
+       forw = curr->next;
+     }
+   }
+   //adding the backlist to mainlist again.
+   curr = head;
+   while ( curr->next != nullptr ) {
+     curr = curr->next;
+   }
+   curr->next = backHead;
+ }
+
+ int main() {
+  Node * head = nullptr;
+  insert( head, 1 );
+  insert( head, 2 );
+  insert( head, 3 );
+  insert( head, 4 );
+  insert( head, 5 );
+  insert( head, 6 );
+  std::cout << "List before rearrange: ";
+  printList( head );
+  rearrange( head );
+  std::cout << "List after  rearrange: ";
+  printList( head );
+
+  Node * head2 = nullptr;
+  insert( head2, 12 );
+  insert( head2, 14 );
+  insert( head2, 16 );
+  insert( head2, 18 );
+  insert( head2, 20 );
+  std::cout << "List before rearrange: ";
+  printList( head2 );
+  rearrange( head2 );
+  std::cout << "List after  rearrange: ";
+  printList( head2 );
+
+  return 0;
+ }