21. Merge Two Sorted Lists

1. Description

You are given the heads of two sorted linked lists list1 and list2.
Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.
Return the head of the merged linked list.

2. Example

Example 1

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2

Input: list1 = [], list2 = []
Output: []

Example 3

Input: list1 = [], list2 = [0]
Output: [0]

3. Constraints

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

4. Solutions

Linked List

n is the sum of nodes in list1 and list2
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *list1, ListNode *list2) {
        ListNode dummy;
        ListNode *iter = &dummy;
        while (list1 != nullptr && list2 != nullptr) {
            if (list1->val <= list2->val) {
                iter->next = list1;
                list1 = list1->next;
            } else {
                iter->next = list2;
                list2 = list2->next;
            }

            iter = iter->next;
        }

        iter->next = list1 == nullptr ? list2 : list1;

        return dummy.next;
    }
};