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。

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
//合并到root.next
public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if(list1 == null){
            return list2;
        }
        if(list2 == null){
            return list1;
        }
        
        ListNode root = new ListNode(0);
        ListNode pointer = root;
        
        while(list1 != null && list2 != null){
            if(list1.val <= list2.val){
                pointer.next = list1;
                list1 = list1.next;
            }else{
                pointer.next = list2;
                list2 = list2.next;
            }
            pointer = pointer.next;
        }
        
        if(list1 != null){
            pointer.next = list1;
        }
        
        if(list2 != null){
            pointer.next = list2;
        }
        
        return root.next;
    }
}