LeetCode #21 Merge Two Sorted Lists

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Problem

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

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

Solution1

Method: Recursive

Time Complexity:

Space Complexity:

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

    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1 or not l2:
            return l1 or l2
    
        if l1.val < l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)
            return l2

Solution2

Method: Iterative

Time Complexity:

Space Complexity:

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

    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        dummy = cur = ListNode(0)
    
        while l1 and l2:
            if l1.val < l2.val:
                cur.next = l1
                l1 = l1.next
            else:
                cur.next = l2
                l2 = l2.next
    
            cur = cur.next
    
        cur.next = l1 or l2
    
        return dummy.next

Solution3

Method: In-place Iterative

Time Complexity:

Space Complexity:

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

    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if None in (l1, l2):
            return l1 or l2
    
        dummy = cur = ListNode(0)
        dummy.next = l1
    
        while l1 and l2:
            if l1.val < l2.val:
                l1 = l1.next
            else:
                nxt = cur.next
                cur.next = l2
                tmp = l2.next
                l2.next = nxt
                l2 = tmp
            cur = cur.next
        cur.next = l1 or l2
        return dummy.next

or

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1:
            return l2
        if not l2:
            return l1

        least, second = (l1, l2) if l1.val <= l2.val else (l2, l1)
        head = least

        while least.next and second:
            if least.next.val >= second.val:
                tmp = least.next
                least.next = second
                second = tmp

            least = least.next

        if second:
            least.next = second

        return head

Reference

https://leetcode.com/problems/merge-two-sorted-lists/discuss/9735/Python-solutions-(iteratively-recursively-iteratively-in-place).