Day-45: Rearrange linkedlist inplace

Author: mandliya

README.md

@@ -4,9 +4,9 @@
 
 | Current Status|     Stats     |
 | :------------: | :----------: |
-| Total Problems | 63 |
-| Current Streak | 44 |
-| Longest Streak | 44 ( August 17, 2015 - September 29, 2015 ) |
+| Total Problems | 64 |
+| Current Streak | 45 |
+| Longest Streak | 45 ( August 17, 2015 - September 30, 2015 ) |
 
 </center>
 
@@ -27,6 +27,7 @@
 | Given a sorted linked list with duplicates, remove duplicates in one iteration. | [removeDuplicatesFromSortedList.cpp](linked_list_problems/removeDuplicatesFromSortedList.cpp)| 
 | Find the nth node of linked list from last. | [nthToLastNode.cpp] (linked_list_problems/nthToLastNode.cpp) |
 | Sort a linked list using merge sort | [merge_sort.cpp] (linked_list_problems/merge_sort.cpp) |
+| Given a singly linked list L<sub>0</sub> -> L<sub>1</sub> -> … -> L<sub>n-1</sub> -> L<sub>n</sub>. Rearrange the nodes in the list (in place) so that the new formed list is : L<sub>0</sub> -> L<sub>n</sub> -> L<sub>1</sub> -> L<sub>n-1</sub> -> L<sub>2</sub> -> L<sub>n-2<sub> ....| [rearrange_list.cpp](linked_list_problems/rearrange_list.cpp)|
 
 ### Include 
 Include contains single header implementation of data structures and some algorithms.

linked_list_problems/rearrange_list.cpp

@@ -0,0 +1,98 @@
+/**
+ * Given a singly linked list L0 -> L1 -> … -> Ln-1 -> Ln.
+ * Rearrange the nodes in the list so that the new formed list is : L0 -> Ln -> L1 -> Ln-1 -> L2 -> Ln-2 …
+ * Example:
+ * Input:  1 -> 2 -> 3 -> 4
+ * Output: 1 -> 4 -> 2 -> 3
+ * Input:  1 -> 2 -> 3 -> 4 -> 5
+ * Output: 1 -> 5 -> 2 -> 4 -> 3 
+ */
+
+#include <iostream>
+
+struct Node {
+  int data;
+  Node * next;
+  Node( int d )
+    : data{ d }, next{ nullptr } { }
+};
+
+void printList( Node * node )
+{
+  while( node ) {
+    std::cout << node->data << "-->";
+    node = node->next;
+  }
+  std::cout << "NULL" << std::endl;
+}
+
+void reverse( Node * & head )
+{
+  Node * nextNode = nullptr;
+  Node * newHead = nullptr;
+  Node * curr = head;
+  while( curr ) {
+    nextNode = curr->next;
+    curr->next = newHead;
+    newHead = curr;
+    curr = nextNode;
+  }
+  head = newHead;
+}
+
+Node * middleNode( Node * head )
+{
+  Node * slowPtr = head;
+  Node * fastPtr = head;
+  while( fastPtr && fastPtr->next ) {
+    fastPtr = fastPtr->next->next;
+    slowPtr = slowPtr->next;
+  }
+  return slowPtr;
+}
+
+void rearrange( Node * & head )
+{
+  //step 1 split the list in two lists first half and second half
+  Node * midNode = middleNode(head);
+  Node * itr1 = head;
+  Node * itr2 = midNode->next;
+  midNode->next = nullptr;
+
+  //step 2 reverse the second half
+  reverse(itr2);
+
+  //step 3 - merge the alternate nodes of these two list in one list.
+  //dummy node as head
+  Node * newHead = new Node(0);
+  Node * curr = newHead;
+  while( itr1 || itr2 ) {
+    if (itr1) {
+      curr->next = itr1;
+      curr = curr->next;
+      itr1 = itr1->next;
+    }
+    if (itr2) {
+      curr->next = itr2;
+      curr = curr->next;
+      itr2 = itr2->next;
+    }
+  }
+
+  //update the head
+  head = newHead->next;
+}
+
+
+int main()
+{
+  Node *head = new Node(1);
+  head->next = new Node(2);
+  head->next->next = new Node(3);
+  head->next->next->next = new Node(4);
+  head->next->next->next->next = new Node(5);
+  printList(head);
+  rearrange(head);
+  printList(head);
+  return 0;
+}