working solution

problem1.py

@@ -0,0 +1,17 @@
+# problem 1 
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+        prev = None
+        while(head):
+            temp = head.next 
+            head.next = prev
+            prev = head
+            head = temp
+        return prev

problem2.py

@@ -0,0 +1,20 @@
+# problem 2 
+
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        temp_head = ListNode(-1)
+        temp_head.next = head
+        slow_ptr = temp_head
+        fast_ptr = temp_head
+        for _ in range(n+1):
+            fast_ptr = fast_ptr.next
+        while(fast_ptr):
+            fast_ptr = fast_ptr.next
+            slow_ptr = slow_ptr.next
+        slow_ptr.next = slow_ptr.next.next 
+        return temp_head.next

problem3.py

@@ -0,0 +1,28 @@
+# problem 3 
+
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+
+class Solution:
+    def detectCycle(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+        slow = head 
+        fast = head
+        match_flag = False
+        while(fast.next and fast.next.next):
+            fast=fast.next.next 
+            slow=slow.next 
+            if fast == slow:
+                match_flag = True 
+                break
+        if not match_flag:
+            return None
+        slow = head
+        while slow!=fast:
+            slow = slow.next
+            fast = fast.next
+        return slow