leetcode-链表-21.合并 两个有序链表

方法 1：递归

class Solution 
{
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2)
	{
        if(l1 == NULL) return l2;
        else if (l2 == NULL) return l1;
        else if (l1->val < l2->val) 
		{
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }
		else 
		{
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
    }
};

方法2：迭代

class Solution 
{
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) 
	{
        ListNode* prehead = new ListNode(-1);
        ListNode* prev = prehead;
        while(l1 != NULL && l2 != NULL)
		{
            if(l1->val <= l2->val) 
			{
                prev->next = l1;
                l1 = l1->next;
            }
			else 
			{
                prev->next = l2;
                l2 = l2->next;
            }
            prev = prev->next;
        }
        prev->next = l1 != NULL ? l1 : l2;

        return prehead->next;
    }
};