[Leetcode]21:Merge Two Sorted Lists

Link : https://leetcode.com/problems/merge-two-sorted-lists/

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

把兩個排列過後的 list，按順序大小 merge成一個 list。

圖來自leetcode

Thoughts :
一開始想要直接會透過修改指針來做，結果發現會出現不知道要return l1 還 是 l2 的問題，後來才發現可以另外製作一個prev的指針來做就好，之後直接return prev.next就可以。

Solutions :
# iterative solution

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        head = ListNode(0)
        prev = head
        while l1 and l2:
            # 假設l1的node小於等於l2的node的話
            if(l1.val <= l2.val):
                #直接把prev的下一個接上，就沒有頭的問題
                prev.next =l1
                l1 = l1.next
            else: #l1.val>l2.val 
                prev.next = l2
                l2 = l2.next
            # prev往下接
            prev = prev.next
        # 如果接下來 l1 是 None
        if l1 == None:
            # 直接接l2
            prev.next = l2
        # 如果l2接下來是 None
        elif l2 == None:
            # 直接接l1
            prev.next = l1
        return head.next

# Recursive Solution

Time Complexity : O(n+m)?, Space Complexity : O(1)?