21. Merge Two Sorted Lists

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

先设置空的哨兵节点，然后将尾部指针指向这个节点。
遍历两个链表，将尾部节点的下一个值指向两个节点中值较小的一个。
然后将指针移动到下一个值。
最后返回哨兵节点的下一个节点。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummyHead = new ListNode();
        ListNode tail = dummyHead;

        
        while ( list1 != null && list2 !=null ){
            if (list1.val < list2.val){
                tail.next = list1;
                list1 = list1.next;
                tail = tail.next;
            }
            else{
                tail.next = list2;
                list2 = list2.next;
                tail = tail.next;
            }
 
        }
        
        if ( list1 == null){
            tail.next = list2;
        }
        else {
            tail.next = list1;
        }
        
        return dummyHead.next;
    }
}