# 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]:
        # 前一個指標
        prev = None
        # 當前指標
        curr = head
        # 遍歷串列
        while curr:
            # 暫存下個指標
            temp_next = curr.next
            # 將當前指標指向反轉
            curr.next = prev
            # 將前個指標指向當前指標
            prev = curr
            # 將當前指標向後移動
            curr = temp_next
        # 回傳移動到新頭部的 prev
        return prev