Added java dsa code by Ankan Sen

DeleteMiddleNode.md

@@ -0,0 +1,40 @@
+package com.thealgorithms.linkedlist;
+
+public class DeleteMiddleNode {
+
+    public static ListNode deleteMiddle(ListNode head) {
+        if (head == null || head.next == null) return null;
+
+        ListNode slow = head, fast = head, prev = null;
+        while (fast != null && fast.next != null) {
+            prev = slow;
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        prev.next = slow.next; // remove middle node
+        return head;
+    }
+
+    public static void printList(ListNode head) {
+        while (head != null) {
+            System.out.print(head.val + " -> ");
+            head = head.next;
+        }
+        System.out.println("null");
+    }
+
+    public static void main(String[] args) {
+        ListNode head = new ListNode(1);
+        head.next = new ListNode(2);
+        head.next.next = new ListNode(3);
+        head.next.next.next = new ListNode(4);
+        head.next.next.next.next = new ListNode(5);
+
+        System.out.print("Original list: ");
+        printList(head);
+
+        head = deleteMiddle(head);
+        System.out.print("After deleting middle: ");
+        printList(head);
+    }
+}

DetectCycleInLL.md

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+package com.thealgorithms.linkedlist;
+
+class ListNode {
+    int val;
+    ListNode next;
+    ListNode(int val) { this.val = val; this.next = null; }
+}
+
+public class DetectCycle {
+
+    public static boolean hasCycle(ListNode head) {
+        ListNode slow = head, fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+            if (slow == fast) {
+                return true; // cycle detected
+            }
+        }
+        return false; // no cycle
+    }
+
+    public static void main(String[] args) {
+        ListNode head = new ListNode(3);
+        head.next = new ListNode(2);
+        head.next.next = new ListNode(0);
+        head.next.next.next = new ListNode(-4);
+        head.next.next.next.next = head.next; // create a cycle
+
+        System.out.println("Cycle exists: " + hasCycle(head));
+    }
+}

FindMiddleNode.md

@@ -0,0 +1,24 @@
+package com.thealgorithms.linkedlist;
+
+public class FindMiddleNode {
+
+    public static ListNode findMiddle(ListNode head) {
+        ListNode slow = head, fast = head;
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+        return slow; // middle node
+    }
+
+    public static void main(String[] args) {
+        ListNode head = new ListNode(1);
+        head.next = new ListNode(2);
+        head.next.next = new ListNode(3);
+        head.next.next.next = new ListNode(4);
+        head.next.next.next.next = new ListNode(5);
+
+        ListNode middle = findMiddle(head);
+        System.out.println("Middle node value: " + middle.val);
+    }
+}

Sort_0s_1s_2s.md

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+package com.thealgorithms.linkedlist;
+
+import java.util.Arrays;
+
+public class Sort012 {
+
+    public static void sortColors(int[] nums) {
+        int low = 0, mid = 0, high = nums.length - 1;
+
+        while (mid <= high) {
+            switch (nums[mid]) {
+                case 0 -> {
+                    int temp = nums[low];
+                    nums[low] = nums[mid];
+                    nums[mid] = temp;
+                    low++;
+                    mid++;
+                }
+                case 1 -> mid++;
+                case 2 -> {
+                    int temp = nums[mid];
+                    nums[mid] = nums[high];
+                    nums[high] = temp;
+                    high--;
+                }
+            }
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] nums = {2, 0, 2, 1, 1, 0};
+        sortColors(nums);
+        System.out.println(Arrays.toString(nums));
+    }
+}