LeetCode-in-Cpp

21. Merge Two Sorted Lists

Easy

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

/**
 * 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* l1, ListNode* l2) {
        ListNode dummy(-1);
        ListNode* list = &dummy;
        
        while (l1 != nullptr || l2 != nullptr) {
            if (l1 != nullptr && l2 != nullptr) {
                if (l1->val <= l2->val) {
                    list->next = new ListNode(l1->val);
                    l1 = l1->next;
                } else {
                    list->next = new ListNode(l2->val);
                    l2 = l2->next;
                }
            } else if (l1 != nullptr) {
                list->next = new ListNode(l1->val);
                l1 = l1->next;
            } else {
                list->next = new ListNode(l2->val);
                l2 = l2->next;
            }
            list = list->next;
        }
        
        return dummy.next;
    }
};

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