递归和迭代，C++ ，思路清晰

主要思路：
链表的天性是能够很好的支持插入，递归和非递归思路其实差不多，都是逐个比较。不过我建议不太了解的童鞋可以先看分递归的，把思路看明白递归的就容易了

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if (pHead1 == nullptr)
            return pHead2;
        if (pHead2 == nullptr)
            return pHead1;
        if (pHead1->val<pHead2->val){
            pHead1->next = Merge(pHead1->next,pHead2);
            return pHead1;
        }else{
            pHead2->next = Merge(pHead2->next,pHead1);
            return pHead2;
        }

    }
};

//迭代
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if (pHead1== nullptr)
            return pHead2;
        if (pHead2==nullptr)
            return pHead1;
        ListNode* mergeHead = nullptr;

        if (pHead1->val<=pHead2->val){
            mergeHead= pHead1;
            pHead1 = pHead1->next;
        }else{
            mergeHead= pHead2;
            pHead1 = pHead2->next;
        }
        ListNode* cur = mergeHead;
        while (pHead1 && pHead2){
            if (pHead1->val<=pHead2->val){
                cur->next =pHead1;
                pHead1 = pHead1->next;
            } else{
                cur->next = pHead2;
                pHead2 = pHead2->next;
            }
            cur = cur->next;
        }
        if(pHead1)
            cur->next = pHead1;
        if (pHead2)
            cur->next = pHead2;

        return mergeHead;
    }