LeetCode 21. Merge Two Sorted Lists

Linked List

Merge two sorted linked lists and return it as a sorted list. The 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.

Solution:

English Version in Youtube

中文版解答Youtube Link

中文版解答Bilibili Link

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode *head = new ListNode(-1);
        ListNode *fake_head = head;
        
        while (l1 != nullptr || l2 != nullptr) {
            if (l1 == nullptr) {
                head->next = l2;
                break;
            } else if(l2 == nullptr) {
                head->next = l1;
                break;
            }
            
            if (l1->val <= l2->val) {
                head->next = l1;
                head = l1;
                l1 = l1->next;
            } else {
                head->next = l2;
                head = l2;
                l2 = l2->next;
            }
        }
        return fake_head->next;
    }
};