Linked List -1

Problem1.py

@@ -0,0 +1,18 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+
+# Time Complexity --> O(n)
+# Space Complexity --> O(1)
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        curr = head
+        while curr is not None:
+            temp = curr.next
+            curr.next = prev
+            prev = curr
+            curr = temp
+        return prev

Problem2.py

@@ -0,0 +1,27 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+
+# Time Complexity --> O(n)
+# Space Complexity --> O(1)
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        dummy = ListNode(None)
+        dummy.next = head
+        slow, fast = dummy, dummy
+
+        count = 0
+        while count<=n:
+            count += 1
+            fast = fast.next
+
+        while fast is not None:
+            fast = fast.next
+            slow = slow.next
+        temp = slow.next
+        slow.next = slow.next.next
+        temp.next = None
+
+        return dummy.next

Problem3.py

@@ -0,0 +1,26 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+# Time Complexity --> O(n)
+# Space Complexity --> O(1)
+class Solution:
+    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        slow = head
+        fast = head
+
+        while fast is not None and fast.next is not None:
+            slow = slow.next
+            fast = fast.next.next
+            if slow == fast:
+                break
+
+        if fast is None or fast.next is None:
+            return None
+        fast = head
+        while slow!=fast:
+            slow = slow.next
+            fast = fast.next
+        return slow