LeetCode热题100-反转链表

python3实现

• 循环方法：设置双指针，空间复杂度1，时间复杂度n

# 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]:
        cur, pre = head, None
        while cur:
            tmp = cur.next
            cur.next = pre
            pre = cur
            cur = tmp
        return pre

这里返回的pre，因为结束循环的条件为cur为空，当cur为空时，pre正好为表头。

• 递归：注意递归设置的条件，第一次返回的表头

# 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]:
        def recur(cur, pre):
            if not cur:
                return pre
            res = recur(cur.next, cur)
            cur.next = pre
            return res
        
        return recur(head, None)