Day 3 - Linked List

LinkedList.java

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+import utils.ListNode;
+
+public class LinkedList {
+    public static void main(String[] args) {
+    }
+
+    /**
+     * Reverse Linked List (iterative)
+     * <p>
+     * Reverse a singly-linked list iteratively.
+     */
+    public ListNode reverse(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+        ListNode cur = head;
+        ListNode prev = null;
+
+        while (cur != null) {
+            ListNode next = cur.next;
+            cur.next = prev;
+            prev = cur;
+            cur = next;
+        }
+        return prev;
+    }
+
+    /**
+     * Reverse Linked List (recursive)
+     * <p>
+     * Reverse a singly-linked list recursively.
+     */
+    public ListNode reverse2(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+
+        ListNode newHead = reverse2(head);
+        head.next.next = head;
+        head.next = null;
+
+        return newHead;
+    }
+
+    /**
+     * Middle Node Of Linked List
+     * <p>
+     */
+    public ListNode middleNode(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+
+        ListNode fast = head;
+        while (fast.next != null && fast.next.next != null) {
+            fast = fast.next.next;
+            head = head.next;
+        }
+        return head;
+    }
+
+    /**
+     * Check If Linked List Has A Cycle
+     * <p>
+     * Check if a given linked list has a cycle. Return true if it does, otherwise
+     * return false.
+     */
+    public boolean hasCycle(ListNode head) {
+        if (head == null) {
+            return false;
+        }
+
+        ListNode fast = head;
+        while (fast.next != null && fast.next.next != null) {
+            fast = fast.next.next;
+            head = head.next;
+            if (fast == head) {
+                return true;
+            }
+        }
+        return true;
+    }
+
+    /**
+     * Insert In Sorted Linked List
+     * <p>
+     * Insert a value in a sorted linked list.
+     */
+    public ListNode insert(ListNode head, int value) {
+        if (head == null) {
+            return new ListNode(value);
+        }
+        ListNode dummy = new ListNode(0);
+        dummy.next = head;
+        ListNode cur = dummy;
+        while (cur.next != null && cur.next.value < value) {
+            cur = cur.next;
+        }
+        ListNode node = new ListNode(value);
+        node.next = cur.next;
+        cur.next = node;
+
+        return dummy.next;
+    }
+
+    /**
+      * Merge Two Sorted Linked Lists
+      * <p>
+      * Merge two sorted lists into one large sorted list.
+    */
+    public ListNode merge(ListNode one, ListNode two) {
+        ListNode dummy = new ListNode(0);
+        ListNode cur = dummy;
+        while (one != null && two != null) {
+            if (one.value < two.value) {
+                cur.next = one;
+                one = one.next;
+            } else {
+                cur.next = two;
+                two = two.next;
+            }
+            cur = cur.next;
+        }
+
+        if (one != null) {
+            cur.next = one;
+        } else {
+            cur.next = two;
+        }
+
+        return dummy.next;
+    }
+}

utils/ListNode.java

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+package utils;
+
+public class ListNode {
+    public int value;
+    public ListNode next;
+
+    public ListNode(int value) {
+        this.value = value;
+    }
+}