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。

方法1：递归

public class Solution {    public ListNode Merge(ListNode list1,ListNode list2) {        if(list1==null){            return list2;        }        if(list2==null){            return list1;        }        if(list1.val<=list2.val){            list1.next=Merge(list1.next,list2);            return list1;        }else{            list2.next=Merge(list1,list2.next);            return list2;        }    }}

方法2：循环

public class Solution {    public ListNode Merge(ListNode list1,ListNode list2) {        if(list1==null){            return list2;        }        if(list2==null){            return list1;        }        ListNode mergeList=null;        ListNode current=null;        while(list1!=null&&list2!=null){            if(list1.val<=list2.val){                if(mergeList==null)                    mergeList=current=list1;                else{                    current.next=list1;                    current=current.next;                }                list1=list1.next;            }else{                if(mergeList==null)                    mergeList=current=list2;                else{                    current.next=list2;                    current=current.next;                }                list2=list2.next;            }        }        if(list1==null){            current.next=list2;        }else{            current.next=list1;        }        return mergeList;    }}