Create MiddleOfLinkedList.java

src/main/java/com/thealgorithms/datastructures/lists/MiddleOfLinkedList.java

@@ -0,0 +1,88 @@
+// Definition for singly-linked list node
+class ListNode {
+    int val;          // Value stored in the node
+    ListNode next;    // Pointer to the next node
+
+    // Constructor to initialize node with a value
+    ListNode(int val) {
+        this.val = val;
+        this.next = null;
+    }
+}
+
+public class MiddleOfLinkedList {
+
+    /**
+     * Finds the middle node of a singly linked list.
+     * If there are two middle nodes, returns the second one.
+     */
+    public ListNode middleNode(ListNode head) {
+        // If the list is empty, just return null
+        if (head == null) return head;
+
+        // Initialize two pointers: slow and fast
+        ListNode slow = head; // moves one step at a time
+        ListNode fast = head; // moves two steps at a time
+
+        // Traverse the list
+        // When 'fast' reaches the end, 'slow' will be at the middle
+        while (fast != null && fast.next != null) {
+            slow = slow.next;         // move slow by one node
+            fast = fast.next.next;    // move fast by two nodes
+        }
+
+        // When loop ends, slow is pointing at the middle node
+        return slow;
+    }
+
+    /**
+     * Helper method to create a linked list from an array.
+     * Example: [1,2,3,4] → 1 → 2 → 3 → 4
+     */
+    public static ListNode createList(int[] values) {
+        // If the array is empty, return null
+        if (values.length == 0) return null;
+
+        // Create the head node
+        ListNode head = new ListNode(values[0]);
+        ListNode current = head;
+
+        // Loop through the rest of the array to build the list
+        for (int i = 1; i < values.length; i++) {
+            current.next = new ListNode(values[i]); // create next node
+            current = current.next;                 // move pointer forward
+        }
+
+        return head; // return the head of the linked list
+    }
+
+    /**
+     * Helper method to print the linked list starting from any node.
+     * Example: prints "3 4 5"
+     */
+    public static void printList(ListNode node) {
+        while (node != null) {
+            System.out.print(node.val + " "); // print current node value
+            node = node.next;                 // move to next node
+        }
+        System.out.println(); // print newline after list
+    }
+
+    public static void main(String[] args) {
+        // Create an instance of this class to access non-static methods
+        MiddleOfLinkedList sol = new MiddleOfLinkedList();
+
+        // Input array for the linked list
+        int[] values = {1, 2, 3, 4, 5};  // Odd-length list
+
+        // Create linked list from array
+        ListNode head = createList(values);
+
+        // Find middle node
+        ListNode middle = sol.middleNode(head);
+
+        // Print all nodes from middle to end
+        System.out.print("Middle node and following nodes: ");
+        printList(middle); // Expected output: 3 4 5
+    }
+}