Remove Linked List Elements

Author: volodimyr

203.remove_linked_list_elements/remove.py

@@ -0,0 +1,46 @@
+# 203. Remove Linked List Elements
+# Topics: 'Linked List', 'Recursion'
+# Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.
+
+# Example 1:
+
+# Input: head = [1,2,6,3,4,5,6], val = 6
+# Output: [1,2,3,4,5]
+
+# Example 2:
+
+# Input: head = [], val = 1
+# Output: []
+
+# Example 3:
+
+# Input: head = [7,7,7,7], val = 7
+# Output: []
+
+# Constraints:
+
+#     The number of nodes in the list is in the range [0, 104].
+#     1 <= Node.val <= 50
+#     0 <= val <= 50
+
+
+from typing import Optional
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+class Solution:
+    def removeElements(self, head: Optional[ListNode], val: int) -> Optional[ListNode]:
+        dummy = ListNode()
+        cur = head
+        prev = dummy
+        while cur:
+            if cur.val != val:
+                prev.next = cur
+                prev = prev.next
+            else:
+                prev.next = None
+            cur = cur.next
+        return dummy.next

234.palindrome_linked_list/palindrome.py

@@ -30,19 +30,45 @@ def __init__(self, val=0, next=None):
         self.val = val
         self.next = next
 
-# Memory O(n), Time O(n)
+# Memory O(1), Time O(n)
 class Solution:
-    def isPalindrome(self, head: Optional[ListNode]) -> bool:
-        arr = []
+    def isPalindrome(self, head: Optional[ListNode])->bool:
+        fast = head
+        slow = head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+        
+        # reverse
+        prev = None
+        while slow:
+            next = slow.next
+            slow.next = prev
+            prev = slow
+            slow = next
+        
         cur = head
-        while cur:
-            arr.append(cur.val)
+        while prev:
+            if prev.val != cur.val:
+                return False
+            prev= prev.next
             cur = cur.next
 
-        L, R = 0, len(arr)-1
-        while L < R:
-            if arr[L] != arr[R]:
-                return False
-            L+=1
-            R-=1
-        return True
+        return True
+
+# Memory O(n), Time O(n)
+# class Solution:
+#     def isPalindrome(self, head: Optional[ListNode]) -> bool:
+#         arr = []
+#         cur = head
+#         while cur:
+#             arr.append(cur.val)
+#             cur = cur.next
+        
+#         L, R = 0, len(arr)-1
+#         while L < R:
+#             if arr[L] != arr[R]:
+#                 return False
+#             L+=1
+#             R-=1
+#         return True