21. Merge Two Sorted Lists

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

1. Use Linked List:

//8 ms    14.5 MB
/**
 * 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 = new ListNode();
        ListNode* curr = dummy;
        while(l1 && l2){
            if(l1->val < l2->val){
                curr->next = l1;
                l1 = l1->next;
            } else {
                curr->next = l2;
                l2 = l2->next;
            }
            curr = curr->next;
        }
        ListNode* rest = l1 ? l1 : l2;
        if(rest)
            curr->next = rest;
        return dummy->next;
    }
};