21. 合并两个有序链表

将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例 1：

输入：l1 = [1,2,4], l2 = [1,3,4]

方法一：递归

/**
 * 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) {
        if(list1 == null){
            return list2;
        }
        if (list2 == null){
            return list1;
        }
        else if (list1.val < list2.val){
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;
        } else {
            list2.next = mergeTwoLists(list1,list2.next);
            return list2;
        }
    }
}

方法二：迭代

/**
 * 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 listNode = new ListNode(-1);

        ListNode pre = listNode;
        while(list2 != null && list1 != null){
            if (list1.val < list2.val){
                pre.next = list1;
                list1 = list1.next;
            }
            else {
                pre.next = list2;
                list2 = list2.next;
            }
            pre = pre.next;
        }
        pre.next = list1 == null? list2:list1;
        return listNode.next;
    }
}