21. Merge Two Sorted Lists

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

Example 1:

Input: l1 = [1,2,4], l2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: l1 = [], l2 = []
Output: []

Example 3:

Input: l1 = [], l2 = [0]
Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both l1 and l2 are sorted in non-decreasing order.

# @lc code=start
using LeetCode

function merge_two_lists(
    list1::Union{ListNode,Nothing}, list2::Union{ListNode,Nothing}
)::Union{ListNode,Nothing}
    isnothing(list1) && isnothing(list2) && return nothing
    node = fake_head = ListNode()
    while !isnothing(list1) && !isnothing(list2)
        node.next = ListNode()
        node = next(node)
        if val(list1) < val(list2)
            node.val = list1.val
            list1 = next(list1)
        else
            node.val = list2.val
            list2 = next(list2)
        end
    end
    l = isnothing(list2) ? list1 : list2
    while !isnothing(l)
        node.next = ListNode()
        node = next(node)
        node.val = l.val
        l = next(l)
    end
    return next(fake_head)
end

# @lc code=end

merge_two_lists (generic function with 1 method)

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