Add Odd-Even Linked List reordering algorithm (two-pointer approach)

dsa_in_js/linkedlist/odd_even_linked_list.js

@@ -0,0 +1,76 @@
+/**
+ * Definition for singly-linked list.
+ */
+export class ListNode {
+  /**
+   * Creates a new ListNode.
+   * @param {number} value - The value of the node
+   * @param {ListNode|null} next - The reference to the next node (default is null)
+   */
+  constructor(value = 0, next = null) {
+    this.value = value;
+    this.next = next;
+  }
+}
+
+/**
+ * Groups all nodes with odd indices together followed by even indices.
+ * @param {ListNode} head - The head of the linked list
+ * @returns {ListNode} - The reordered linked list
+ */
+export function oddEvenLinkedList(head) {
+  if (!head || !head.next) return head;
+
+  let odd = head;
+  let even = head.next;
+  const evenHead = even;
+
+  while (even && even.next) {
+    odd.next = even.next;
+    odd = odd.next;
+    even.next = odd.next;
+    even = even.next;
+  }
+
+  // Link the end of odd list to the start of even list
+  odd.next = evenHead;
+
+  return head;
+}
+
+/**
+ * Helper function to print the linked list (for testing purposes).
+ * @param {ListNode} head - The head of the linked list
+ */
+export function printList(head) {
+  const values = [];
+  let current = head;
+  while (current) {
+    values.push(current.value);
+    current = current.next;
+  }
+  console.log(values.join(" -> "));
+}
+
+// Tiny self-check in browser/console environment
+if (typeof window !== "undefined")
+  (function selfTest() {
+    const head = new ListNode(1,
+      new ListNode(2,
+        new ListNode(3,
+          new ListNode(4,
+            new ListNode(5)
+          )
+        )
+      )
+    );
+
+    console.log("Original list:");
+    printList(head);
+    console.log("---");
+
+    const reordered = oddEvenLinkedList(head);
+    console.log("Reordered (odd-even) list:");
+    printList(reordered);
+    console.log("---");
+  })();