Problem

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Solution

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *l3 = new ListNode(-1);
        ListNode *t3 = l3;
        
        while(l1 && l2){
            if(l1->val <= l2->val){
                t3->next = l1;
                l1 = l1->next;
            }else{
                t3->next = l2;
                l2 = l2->next;
            }
            t3 = t3->next;
        }
        
        t3->next = l1? l1 : l2;
        //ListNode *tmp = l3;
        l3 = l3->next;
        //delete tmp;
        return l3;
    }
};

Analysis

Time complexity: O(n) where n is the length of the linked lists
Space complexity: O(n)