題目描述 輸入兩個單調遞增的鏈表，輸出兩個鏈表合成后的鏈表，當然我們需要合成后的鏈表滿足單調不減規則。

class Solution {
public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if(pHead1 == NULL && pHead2 == NULL)
            return NULL;
        if(pHead1 == NULL){
            return pHead2;
        }
        if(pHead2 == NULL){
            return pHead1;
        }
        ListNode *Head = NULL;
        ListNode *p = NULL;
        
        while(pHead1 && pHead2){
            if(pHead1->val <= pHead2->val){
                if(Head == NULL){
                    Head = p = pHead1;
                }else{
                     p->next = pHead1;
                     p = p->next;
                }
                pHead1 = pHead1->next;
            }
            //這里必須是else，因為第一步當中指針操作之后可能也滿足第二個if，所以兩個if不行
            else if(pHead1->val > pHead2->val){
                if(Head == NULL){
                    Head = p = pHead2;
                }else{
                     p->next = pHead2;
                     p = p->next;
                }
                pHead2 = pHead2->next;
                
            }
        }
            if(pHead1 == NULL){
                p->next = pHead2;
            }
            if(pHead2 == NULL){
                p->next = pHead1;
            }
        return Head;
    }
};

　　記錄一下踩的坑。注釋處必須是else，如果倆if就報錯