---
tags: leetcode
---

# [21. Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)
---
# My Solution
## The Key Idea for Solving This Coding Question
## C++ Code

/**
 * Definition for singly-linked list.
 * 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) {}
 * };
 */


class Solution {
public:
    ListNode *mergeTwoLists(ListNode *list1, ListNode *list2) {
        ListNode *dummy = new ListNode(), *it = dummy;

        while (list1 != nullptr && list2 != nullptr) {
            if (list1->val < list2->val) {
                it->next = list1;
                list1 = list1->next;
            } else {
                it->next = list2;
                list2 = list2->next;
            }
            it = it->next;
        }

        if (list1 != nullptr) {
            it->next = list1;
        } else if (list2 != nullptr) {
            it->next = list2;
        }

        ListNode *head = dummy->next;
        delete dummy;

        return head;
    }
};

## Time Complexity
$O(n1 + n2)$
$n1$ is the number of nodes in `l1`.
$n2$ is the number of nodes in `l2`.
## Space Complexity
$O(1)$
# Miscellaneous
<!--
# Test Cases

[1,2,4]
[1,3,4]

[]
[]

[]
[0]

[2,4,6]
[1,3,5,7,9,11]

[4,5,8,9,12,14]
[8,12,14]

[1]
[]

[]
[1,2,4,5,6,7]

[2,4,6,8,9]
[]

[1,11,21,31,41,51,61,71,81,91]
[2,22,32]

-->