Create linkedlist-1.java

linkedlist-1.java

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+
+// Approach:
+// 1. Initialize two pointers: prev as null and curr as head.
+// 2. Traverse the linked list while curr is not null.
+// 3. Store the next node, reverse the current node's pointer,
+//    then move prev and curr one step forward.
+// 4. Continue until all links are reversed.
+// 5. Return prev as the new head of the reversed linked list.
+
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        if(head==null) return head;
+        ListNode prev=null;
+        ListNode curr=head;
+        while(curr!=null){
+            ListNode temp=curr.next;
+            curr.next=prev;
+            prev=curr;
+            curr=temp;
+        }
+        return prev;
+    }
+}
+//problem-2
+// Approach:
+// 1. Create a dummy node pointing to the head to handle edge cases.
+// 2. Place two pointers (fast and slow) at the dummy node.
+// 3. Move the fast pointer (n + 1) steps ahead to maintain a gap of n nodes.
+// 4. Move both pointers together until fast reaches the end.
+// 5. The slow pointer will be just before the node to be deleted.
+// 6. Remove the target node by updating the next pointer.
+// 7. Return dummy.next as the updated head.
+
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+class Solution {
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        ListNode dummynode=new ListNode(-1);
+        if(head==null) return head;
+        dummynode.next=head;
+        int cnt=0;
+        ListNode slow=dummynode;
+        ListNode fast=dummynode;
+        while(cnt<=n){
+            fast=fast.next;
+            cnt++;
+        }
+        while(fast!=null){
+            slow=slow.next;
+            fast=fast.next;
+        }
+        ListNode temp=slow.next;
+        slow.next=slow.next.next;
+        temp=null;
+        return dummynode.next;
+    }
+}
+//problem 3
+// Approach:
+// 1. Traverse the linked list while storing each visited node in a HashSet.
+// 2. Before adding a node, check if it already exists in the set.
+// 3. If it exists, that node is the starting point of the cycle.
+// 4. If the traversal reaches null, no cycle exists.
+// 5. Return the cycle's starting node or null if there is no cycle.
+
+// Time Complexity: O(n)
+// Space Complexity: O(n)
+public class Solution {
+    public ListNode detectCycle(ListNode head) {
+        HashSet<ListNode> map=new HashSet<>();
+        if(head==null) return head;
+        ListNode curr=head;
+        while(curr!=null){
+            if(map.contains(curr)){
+                return curr;
+            }
+            map.add(curr);
+            curr=curr.next;
+        }
+        return null;
+    }
+}