clone linked list with two approaches

Author: Arjiit

Two Pointers/cloneLinkedlist.java

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+/*
+// Definition for a Node.
+class Node {
+    int val;
+    Node next;
+    Node random;
+
+    public Node(int val) {
+        this.val = val;
+        this.next = null;
+        this.random = null;
+    }
+}
+*/
+
+// O(N) time, O(N) space
+class Solution {
+    public Node copyRandomList(Node head) {
+        if (head == null) {
+            return null;
+        }
+        
+        // mapping original node to its clone
+        HashMap<Node, Node> hm = new HashMap<>();
+        
+        // giving all nodes their clone in the mapping
+        Node cur = head;
+        while(cur != null) {
+            hm.put(cur, new Node(cur.val)); //(node, node_clone)
+            cur = cur.next;
+        }
+        
+        //resetting cur pointer to head of the original list
+        // giving all clones their next and random ptr
+        cur = head;
+        while(cur != null) {
+            hm.get(cur).next = hm.get(cur.next);
+            hm.get(cur).random = hm.get(cur.random);
+            cur = cur.next;
+        }
+        return hm.get(head);
+    }
+}
+
+// O(N) time, O(1) space
+
+/*
+// Definition for a Node.
+class Node {
+    int val;
+    Node next;
+    Node random;
+
+    public Node(int val) {
+        this.val = val;
+        this.next = null;
+        this.random = null;
+    }
+}
+*/
+class Solution {
+    public Node copyRandomList(Node head) {
+        Node cur = head;
+        Node next = null;
+        
+        // 1st pass, just connect original nodes to its clone
+        while(cur != null) {
+            next = cur.next; //storing next value of original node, so that we don't lose it
+            Node copyNode = new Node(cur.val);
+            cur.next = copyNode;
+            copyNode.next = next;
+            // advancing to next node in original list
+            cur = next;  
+        }
+        // 2nd pass, assign clone its random mapping
+        cur = head;
+        while(cur != null) {
+            if (cur.random != null) {
+                cur.next.random = cur.random.next; // since we want random's clone
+            }
+            cur = cur.next.next;
+        }
+        //3rd pass, removing the next value of original nodes
+        cur = head;
+        Node dummyHead = new Node(0);
+        Node cloneListTail = dummyHead;
+        Node copy = null;
+        while(cur != null) {
+            next = cur.next.next; //next in the original node
+            // saving cur's clone
+            copy = cur.next; 
+            // appending copy to final list tail
+            cloneListTail.next = copy;
+            cloneListTail = copy;
+            
+            cur.next = next; // restoring original cur's next value
+            
+            cur = next; // advancing curr to next in the original node
+            
+        }
+        return dummyHead.next;
+    }
+}