#include <bits/stdc++.h>
#include <iostream>

using namespace std;

/* 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);
};