输入两个单调递增的链表，输出两个链表合成后的链表，当然我们需要合成后的链表满足单调不减规则

/*struct ListNode {    int val;    struct ListNode *next;    ListNode(int x) :            val(x), next(NULL) {    }};*/class Solution {public:    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)    {        if(pHead1==NULL)               return pHead2;        if(pHead2==NULL)             return pHead1;        ListNode*  node=new  ListNode(NULL);        if(pHead1->val
     
      val)        {            node=pHead1;            node->next=Merge(pHead1->next,pHead2);        }        else            {            node=pHead2;            node->next=Merge(pHead1,pHead2->next);        }        return node;            }};