Merge pull request #141 from WazedKhan/phase-02-linked-list

Phase-02: Linked-List

Author: WazedKhan

prep/practice/linked_list.py

@@ -0,0 +1,103 @@
+from typing import Optional
+
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+def print_list(head):
+    current = head
+    while current is not None:
+        print(current.val, end=" ")
+        current = current.next
+    print()
+
+
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        prev = None
+        current = head
+
+        while current:
+            next_node = current.next
+            current.next = prev
+            prev = current
+            current = next_node
+        return prev
+
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        dummy = ListNode()
+        tail = dummy
+
+        while list1 and list2:
+            if list1.val < list2.val:
+                tail.next = list1
+                list1 = list1.next
+            else:
+                tail.next = list2
+                list2 = list2.next
+            tail = tail.next
+
+        if list1:
+            tail.next = list1
+        else:
+            tail.next = list2
+
+        return dummy.next
+
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        tortoise, hare = head, head
+
+        while hare and hare.next:
+            tortoise = tortoise.next
+            hare = hare.next.next
+            if tortoise == hare:
+                return True
+
+        return False
+
+    def list_len(self, head: Optional[ListNode]) -> int:
+        current = head
+        count = 0
+        while current:
+            count += 1
+            current = current.next
+        return count
+
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        pos = self.list_len(head) - n
+        dummy = ListNode(0)
+        dummy.next = head
+        current = dummy
+
+        for _ in range(pos):
+            current = current.next
+        current.next = current.next.next
+        return dummy.next
+
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        slow, fast = head, head
+
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+        half = slow.next
+        slow.next = None
+
+        prev = None
+        while half:
+            next_node = half.next
+            half.next = prev
+            prev = half
+            half = next_node
+
+        while head and prev:
+            nxt_head = head.next
+            head.next = prev
+            nxt_prev = prev.next
+            prev.next = nxt_head
+
+            head = nxt_head
+            prev = nxt_prev