206. 反转链表 (Easy) - 链表

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        """
        迭代法
        时间: O(n)
        空间: O(1)

        图解：
        原始: 1 -> 2 -> 3 -> None
        步骤1: None <- 1    2 -> 3 -> None
        步骤2: None <- 1 <- 2    3 -> None
        步骤3: None <- 1 <- 2 <- 3
        """
        prev = None
        curr = head

        while curr:
            next_temp = curr.next  # 保存下一个节点
            curr.next = prev       # 反转指针
            prev = curr            # prev 前进
            curr = next_temp       # curr 前进

        return prev