剑指offer 16 合并两个排序的链表

题目描述
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。

//递归
ListNode* Merge(ListNode* pHead1, ListNode* pHead2){
  if(pHead1==NULL) return pHead2;
  if(pHead2==nullptr) return pHead1;
      if(pHead1->val>pHead2->val) {
    pHead2->next=Merge(pHead1, pHead2->next);
    return pHead2;
  }
  if(pHead1->val<pHead2->val) {
    pHead1->next=Merge(pHead1->next, pHead2);
    return pHead1;
  }
  }

// 循环
if(list1 == null){
            return list2;
        }
        if(list2 == null){
            return list1;
        }
        ListNode mergeHead = null;
        ListNode current = null;     
        while(list1!=null && list2!=null){
            if(list1.val <= list2.val){
                if(mergeHead == null){
                   mergeHead = current = list1;
                }else{
                   current.next = list1;
                   current = current.next;
                }
                list1 = list1.next;
            }else{
                if(mergeHead == null){
                   mergeHead = current = list2;
                }else{
                   current.next = list2;
                   current = current.next;
                }
                list2 = list2.next;
            }
        }
        if(list1 == null){
            current.next = list2;
        }else{
            current.next = list1;
        }
        return mergeHead;