合并两个排序的链表

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)
    {
        ListNode* res=NULL;
        if(pHead1==NULL&&pHead2==NULL) return res;
        if(pHead1==NULL){
            res=pHead2;
            return res;
        }
        if(pHead2==NULL){
            res=pHead1;
            return res;
        }
        ListNode* tmp;
        if(pHead1->val>pHead2->val){
             res=pHead2;
            pHead2=pHead2->next;
        }
           
        else{
            res=pHead1;
            pHead1=pHead1->next;
        }
            
        tmp=res;
        while(pHead1!=NULL&&pHead2!=NULL){
            if(pHead1->val<pHead2->val){
                tmp->next=pHead1;
                tmp=tmp->next;
                pHead1=pHead1->next;
            }
            else{
                tmp->next=pHead2;
                tmp=tmp->next;
                pHead2=pHead2->next;
            }
        }
        if(pHead1!=NULL)
            tmp->next=pHead1;
        if(pHead2!=NULL)
            tmp->next=pHead2;
        return res;
    }
};