Linked List

Reverse Linked List

# 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]:
        prev, curr, next = None, head, None
        while curr:
            temp = curr.next
            curr.next = prev
            prev = curr
            curr = temp
        return prev

Merge Two Sorted Linked Lists

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        head1, head2 = list1, list2
        dummy = head = ListNode()
        while head1 and head2:
            if head1.val <= head2.val:
                head.next = head1
                head1 = head1.next
            else:
                head.next = head2
                head2 = head2.next
            head = head.next
        if head1: head.next = head1
        if head2: head.next = head2
        return dummy.next

Linked List Cycle Detection

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        slow, fast = head, head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            if fast == slow: return True
        return False