Merge pull request #675 from Anupam-Panwar/oddEvenLinkedList

Added oddEvenLinkedList.cpp

Author: smv1999

Data Structures/Linked Lists/Singly Linked List/oddEvenLinkedList.cpp

@@ -0,0 +1,120 @@
+/*
+
+Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes 
+with even indices, and return the reordered list.
+
+The first node is considered odd, and the second node is even, and so on.
+
+Note that the relative order inside both the even and odd groups should remain as it was in the input.
+
+You must solve the problem in O(1) extra space complexity and O(n) time complexity.
+
+Input: head = [1,2,3,4,5]
+Output: [1,3,5,2,4]
+
+Constraints:
+ n == number of nodes in the linked list
+ 0 <= n <= 104
+ -106 <= Node.val <= 106
+
+*/
+
+/*
+Time Complexity: O(N), where N is the size of the array
+Space Complexity: O(1), Constant Space
+*/
+
+/*
+
+Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes 
+with even indices, and return the reordered list.
+
+The first node is considered odd, and the second node is even, and so on.
+
+Note that the relative order inside both the even and odd groups should remain as it was in the input.
+
+You must solve the problem in O(1) extra space complexity and O(n) time complexity.
+
+Input: head = [1,2,3,4,5]
+Output: [1,3,5,2,4]
+
+Constraints:
+ n == number of nodes in the linked list
+ 0 <= n <= 104
+ -106 <= Node.val <= 106
+
+*/
+
+/*
+Time Complexity: O(N), where N is the size of the array
+Space Complexity: O(1), Constant Space
+*/
+
+#include <bits/stdc++.h>
+using namespace std;
+
+struct ListNode // Linked list node
+{
+    int val;
+    ListNode *next;
+    ListNode(int x)
+    {
+        val = x;
+        next = NULL;
+    }
+};
+
+ListNode *oddEvenList(ListNode *head)
+{
+    int len = 1;// variable to store length of linked List
+    ListNode *end = head;
+    if (head == NULL)
+        return NULL;
+    while (end->next != NULL) // calculation of length
+    {
+        end = end->next;
+        len++;
+    }
+    if (len <= 2)
+        return head;
+    ListNode *Nend = end; // pointer to point to the new end of the list
+    ListNode *temp = head;
+    while (temp != end)
+    { 
+        ListNode *t = temp->next; 
+        temp->next = t->next; // breaking the link
+        temp = temp->next; 
+        t->next = NULL;
+        Nend->next = t; // adding the node to the end
+        Nend = Nend->next; // updating end
+        if (t == end)
+            break;
+    }
+    return head;
+}
+void printList(ListNode *head) // function to print the list
+{
+    ListNode *temp = head;
+    while (temp != NULL)
+    {
+        cout<<temp->val<<" ";
+        temp = temp->next;
+    }
+    cout<<"\n";
+}
+int main() // driver function
+{
+    int data = 1; // adding the elements to the list
+    ListNode *head = new ListNode(data);
+    ListNode *tail = head;
+    for (int i = 2; i<=5 ; i++)
+    {
+        tail->next = new ListNode(i);
+        tail = tail->next;
+    }
+    
+    head = oddEvenList(head);
+    printList(head);
+
+    return 0;
+}