leetcode刷題筆記（LinkedList相關）

206. Reverse Linked List

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

思路：每步的操作就是：先存儲head的下一個節點，然後讓head指向上一個節點，然後讓q和head分別去向下一個節點。

 

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            return head

        q= None
        while head:
            tmp = head.next
            head.next =q
            q= head
            head = tmp
        return q

 

21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        cur = head = ListNode(0)
        while l1 and l2:
            if l1.val < l2.val:
                head.next = l1
                l1 = l1.next
            else: 
                head.next = l2
                l2 = l2.next
            head =head.next
        head.next = l1 or l2
        return cur.next