chore(CPlusPlus): add reorder list (#373)

Co-authored-by: Arsenic <54987647+Arsenic-ATG@users.noreply.github.com>

algorithms/CPlusPlus/Linked-Lists/Reorder-List.cpp

@@ -0,0 +1,127 @@
+/* Program to reorder a LinkedList in the below given way
+For example: Given 1->2->3->4->5 its reorder is 1->5->2->4->3. */
+#include <iostream>
+using namespace std;
+
+// Linked list Node
+struct Node 
+{
+    int data;
+    struct Node * next;
+
+    Node(int x) 
+    {
+        data = x;
+        next = NULL;
+    }
+};
+
+void reorderList(struct Node * head);
+// Function to create a new Node with given data
+struct Node * newNode(int data) 
+{
+    struct Node * new_Node = new Node(data);
+    new_Node -> data = data;
+    new_Node -> next = NULL;
+    return new_Node;
+}
+
+// Function to printList
+void printList(struct Node * Node) 
+{
+    while (Node != NULL) 
+    {
+        printf("%d ", Node -> data);
+        Node = Node -> next;
+    }
+    printf("\n");
+}
+
+// Function to freeList
+void freeList(struct Node * head) 
+{
+    struct Node * temp;
+    while (head != NULL) 
+    {
+
+        temp = head;
+        head = head -> next;
+        free(temp);
+    }
+}
+
+// driver code
+int main() 
+{
+    int n, m, i, x;
+    struct Node * temp, * head;
+    cout << "Enter the size: ";
+    cin >> n;
+    cin >> x;
+    head = new Node(x);
+    temp = head;
+    for (i = 0; i < n - 1; i++) 
+    {
+        cin >> x;
+        temp -> next = new Node(x);
+        temp = temp -> next;
+    }
+
+    reorderList(head);
+    printList(head);
+    freeList(head);
+    return 0;
+}
+
+// Function to reorderList
+void reorderList(Node * head) 
+{
+    Node * fast = head, * slow = head, * prev = NULL, * t;
+    Node * new_head;
+    if (head -> next != NULL && head != NULL) 
+    {
+        while (fast != NULL && fast -> next != NULL) 
+        {
+            prev = slow;
+            fast = fast -> next -> next;
+            slow = slow -> next;
+        }
+        /* now prev is pointing the middle node of list and fast is pointing the last Node of the list,
+           split the list into two parts and new head will point the 2 part */
+        prev -> next = NULL;
+        new_head = slow;
+        fast = NULL;
+        // will slow is not equal to NULL means the new_head!=empty run the loop and reverse the List
+        while (slow) 
+        {
+            t = slow -> next;
+            slow -> next = fast;
+            fast = slow;
+            slow = t;
+        }
+        new_head = fast;
+        t = head;
+        /* now pick alternate node from both and add it in the right place */
+        while (t) 
+        {
+            slow = new_head;
+            new_head = new_head -> next;
+            slow -> next = t -> next;
+            t -> next = slow;
+            prev = t -> next;
+            t = t -> next -> next;
+        }
+        if (new_head != NULL) 
+        {
+            prev -> next = new_head;
+        }
+    }
+}
+/*
+Input: Enter the size: 5
+5,4,3,2,1
+Output:5,1,4,2,3
+
+Time complexity: O(N)
+Space Complexity: O(1)
+*/

algorithms/CPlusPlus/README.md

@@ -26,6 +26,7 @@
 4. [Circular linked lists](Linked-Lists/circular.cpp)
 5. [Reversing a linked lists](Linked-Lists/reverse.cpp)
 5. [Merging two sorted linked lists](Linked-Lists/merge.cpp)
+6. [Reorder List](Linked-Lists/Reorder-List.cpp)
 
 ## Searching
 1. [Linear Search](Searching/linear-search.cpp)