206_Reverse_Linked_List.py

# 2 Possible Solutions
# 1. Iterative Approach
# 2. Recursive Approach

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    # Iterative
    # Time: O(N), Space: O(1)
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        previousNode = None
        currentNode = head
        
        while currentNode:
            nextNode = currentNode.next
            currentNode.next = previousNode
            previousNode = currentNode
            currentNode = nextNode
        return previousNode
    
    # Recursive
    # Time: O(N), Space: O(N)
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head:
            return None
        
        newHead = head

        if head.next:
            newHead = self.reverseList(head.next)
            head.next.next = head
        head.next = None

        return newHead