LeetCode21.cpp

/**
 * 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 *list1, ListNode *list2)
	{
		if (list1 == NULL and list2 == NULL)
			return NULL;
		else if (list1 == NULL)
			return list2;
		else if (list2 == NULL)
			return list1;
		ListNode *merge = new ListNode();
		ListNode *cur = merge;
		while (list1 != NULL and list2 != NULL)
		{
			if (list1->val <= list2->val)
			{
				merge->next = list1;
				list1 = list1->next;
			}
			else
			{
				merge->next = list2;
				list2 = list2->next;
			}
			merge = merge->next;
		}
		if (list1 == NULL)
		{
			merge->next = list2;
		}
		else
			merge->next = list1;
		return cur->next;
	}
};