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。

/*public class ListNode {    int val;    ListNode next = null;    ListNode(int val) {        this.val = val;    }}*/public class Solution {    public ListNode Merge(ListNode list1,ListNode list2) {                ListNode head = new ListNode(-1);        head.next = null;        ListNode root = head;        while(list1 != null && list2 != null){            if(list1.val < list2.val){                head.next = list1;                 head = list1;                list1 = list1.next;                           }else{                head.next = list2;                 head = list2;                  list2 = list2.next;                           }        }        if(list1 == null && list2 != null){            head.next = list2;        }        if(list2 == null && list1 != null){            head.next = list1;        }                return root.next;    }}