python-leetcode-反转链表

方法1:迭代法

# 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 = None, head

        while curr:
            next_node = curr.next  # 1. 先存下一个节点
            curr.next = prev       # 2. 反转指针
            prev = curr            # 3. prev 前进
            curr = next_node       # 4. curr 前进

        return prev  # prev 成为新头部

方法 2：递归法

# 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]:
        if not head or not head.next:
            return head  # 递归终止条件

        new_head = self.reverseList(head.next)  # 递归反转剩余部分
        head.next.next = head  # 反转当前节点指针
        head.next = None  # 断开原链

        return new_head