Problem statement

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a 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.

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]

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.

Solution: Constructing a new list

For each pair of nodes between the two lists, pick the node having a smaller value to append to the new list.

Code

#include <iostream>
struct ListNode {
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
    if (list1 == nullptr) {
        return list2;
    } else if (list2 == nullptr) {
        return list1;
    }
    // identify which list is head
    ListNode* head = list1;
    if (list2->val < head->val) {
        head = list2;
        list2 = list2->next;
    } else {
        list1 = list1->next;
    }
    ListNode* node = head;
    while (list1 && list2) {
        if (list1->val < list2->val) {
            node->next = list1;
            list1 = list1->next;
        } else {
            node->next = list2;
            list2 = list2->next;
        }
        node = node->next;
    }
    if (list1 == nullptr) {
        node->next = list2;
    } else {
        node->next = list1;
    }
    return head;
}

void printResult(ListNode* head) {
    std::cout << "[";
    while (head) {
        std::cout << head->val << ",";
        head = head->next;
    }
    std::cout << "]\n";
}
int main() {   
    ListNode four1(4);
    ListNode two1(2, &four1);
    ListNode one1(1, &two1);
    ListNode four2(4);
    ListNode three2(3, &four2);
    ListNode one2(1, &three2);
    auto newOne = mergeTwoLists(&one1, &one2);
    printResult(newOne);

    auto empty = mergeTwoLists(nullptr, nullptr);
    printResult(empty);

    ListNode zero(0);
    auto z = mergeTwoLists(nullptr, &zero);
    printResult(z);
}

Output:
[1,1,2,3,4,4,]
[]
[0,]

Complexity

• Runtime: O(N), where N = list1.length + list2.length.

• Extra space: O(1).

References

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

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