C++ 合并两个有序链表

其实合并两个有序链表：最主要的就是内存分配，这是使用的是堆内存，自己控制开辟和释放

遍历式：

代码：

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
		ListNode *head = new ListNode(0);
		ListNode *end = head;
		while(l1 && l2){
			if(l1->val < l2 ->val){
				ListNode *tmp = new ListNode(l1->val);
				end->next = tmp;
				end = tmp;
				l1= l1->next;
			}
			else
			{
				ListNode *tmp = new ListNode(l2->val);
				end->next = tmp;
				end = tmp;
				l2= l2->next;
			}
		}		
		while(l1)
		{
			ListNode *tmp = new ListNode(l1->val);
			end->next = tmp;
			end = tmp;
			l1= l1->next;
		}
		while(l2)
		{
			ListNode *tmp = new ListNode(l2->val);
			end->next = tmp;
			end = tmp;
			l2= l2->next;	
		}
		return head->next;	
    }
    /*
    ListNode* MergeTwoLists(ListNode* l1, ListNode* l2){
    	if(l1->)
    }*/
};

递归式，不用分配内存，但是改变了原有指针的指向：

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