【leetcode】19. 删除链表的倒数第 N 个结点（python）+本地测试

leetcode 提交

方法一：一次遍历（快慢指针）

class Solution:
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        dummy = ListNode(0, head)  # 一次遍历，快慢指针
        slow, fast = dummy, dummy
        while n:
            fast = fast.next
            n -= 1
        while fast.next:
            fast = fast.next
            slow = slow.next
        slow.next = slow.next.next
        return dummy.next

方法二：两次遍历

写法一： 直接删除，注意“头结点”需要特殊处理一下。

class Solution(object):
    def removeNthFromEnd(self, head, n): # 最简单的：两次遍历
        num, p = 0, head
        while p:
            num += 1
            p = p.next
        p = head    
        if num == n and p.next == None: # 仅有一个节点
            return None
        elif num == n:    # 删除头节点
            head = head.next  
        for i in range(num-1):
            if i == num - n - 1:
                q = p.next
                p.next = q.next
            else:
                p = p.next
        return head

写法二：在头结点之前加个哑变量，统一所有结点的操作。

class Solution(object):
    def removeNthFromEnd(self, head, n):
        num, p = 0, head
        while p:
            num += 1
            p = p.next
        dummy = ListNode(0, head)
        p = dummy
        for i in range(1, num-n+1):
            p = p.next
        p.next = p.next.next 
        return dummy.next

本地测试

class ListNode(object):
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
        
def createLinkList(lst):
    head = ListNode(lst[0])
    p = head
    for i in range(1, len(lst)):
        p.next = ListNode(lst[i])
        p = p.next
    return head

def printList(head):
    p = head
    lst = []
    while p:
        lst.append(p.val)
        p = p.next
    return lst

class Solution(object):   # 写法一
    def removeNthFromEnd(self, head, n): # 最简单的：两次遍历
        num, p = 0, head
        while p:
            num += 1
            p = p.next
        p = head    
        if num == n and p.next == None: # 仅有一个节点
            return None
        elif num == n:    # 删除头节点
            head = head.next  
        for i in range(num-1):
            if i == num - n - 1:
                q = p.next
                p.next = q.next
            else:
                p = p.next
        return head

# class Solution(object):   # 写法二
#     def removeNthFromEnd(self, head, n):
#         num, p = 0, head
#         while p:
#             num += 1
#             p = p.next
#         dummy = ListNode(0, head)
#         p = dummy
#         for i in range(1, num-n+1):
#             p = p.next
#         p.next = p.next.next 
#         return dummy.next

# t = [1,2,3,4,5]
# t = [1]
# t = [1,2]
t = [1,2,3,4,5,6]
n = 6
linklist = createLinkList(t)
s = Solution()
s = s.removeNthFromEnd(linklist, n)
printList(s)