adding reverse_linkedlist (solution-1)

W4_6_reverse_linkedlist_three_pointer.py

@@ -0,0 +1,22 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+
+# Time complexity: O(N), we visit all nodes with curr exactly once. 
+# Space complexity: O(1), we just use a few extra pointers (prev, curr, temp)
+
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        curr = head
+
+        while curr != None:
+            temp = curr.next # Store curr.next before overiding.
+            curr.next = prev 
+            prev = curr
+            curr = temp # Moving curr to the next node to process
+
+        return prev
+

W4_7_cycle_linkedlist.py

@@ -0,0 +1,29 @@
+# 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]:
+        slow = head
+        fast = head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+            if slow == fast:
+                start = head
+
+                while start != slow:
+                    start = start.next
+                    slow = slow.next
+
+                return start
+
+        return None
+
+# Time complexity O(N)
+# Space complexity O(1)
+

W4_8_delete_nth_last_node_linkedlist.py

@@ -0,0 +1,43 @@
+# 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]:
+
+        length = 0 
+        curr = head
+        prev = head
+
+        # Get the length of the linkedlist
+        while curr:
+            curr = curr.next
+            length = length + 1
+
+        # Get the position of the node to be deleted
+        del_index = (length - n)
+
+        # for cases with difference 0, removing the head.
+        if del_index == 0:
+            return head.next        
+
+        prev = head
+
+        while del_index > 1:
+            prev = prev.next
+            del_index -= 1
+
+        curr = prev.next
+
+        # Swaping the links to delete targetnode
+        temp = curr.next
+        curr.next = None
+        prev.next = temp
+
+        return head
+
+# Time complexity: O(2N) = O(N)
+# Space complexity: O(1)
+
+