题目：

将两个有序链表合并为一个新的有序链表并返回。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：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

代码：

package leetCode;

import leetCode.TwoHundredAndThirtyFour.ListNode;

/**
 * 2018.7.23
 * 合并两个有序链表
 * @author dhc
 *
 */
public class TwentyOne {
    public class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
        }
    }
    //依次比较两个链表上的值，依次添加到新链表中 9ms
    public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null && l2 != null) {
            return l2;
        }
        if(l1 != null && l2 == null) {
            return l1;
        }
        ListNode re;
        if(l1.val <= l2.val) {
            re = l1;
            l1 = l1.next;

        }else {
            re = l2;
            l2 = l2.next;
        }
        ListNode l = re;
        while(l1 != null && l2 !=null) {
            if(l1.val <= l2.val) {
                l.next = l1;
                l1 = l1.next;
            }else {
                l.next = l2;
                l2 = l2.next;
            }
            l = l.next;
        }
        if(l1 != null) {
            l.next = l1;
        }
        if(l2 != null) {
            l.next = l2;
        }
        return re;
    }
    //贴一个用递归大佬的答案  8ms
    public static ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;

        ListNode head = null;
        if (l1.val <= l2.val){
            head = l1;
            head.next = mergeTwoLists(l1.next, l2);
        } else {
            head = l2;
            head.next = mergeTwoLists(l2.next, l1);
        }
        return head;
    }
    public static void main(String[] args) {
        ListNode head1 = new TwentyOne().new ListNode(1);
        ListNode node11 = new TwentyOne().new ListNode(2);
        ListNode node12 = new TwentyOne().new ListNode(3);
        ListNode head2 = new TwentyOne().new ListNode(3);
        ListNode node21 = new TwentyOne().new ListNode(4);
        ListNode node22 = new TwentyOne().new ListNode(5);
        head1.next = node11;
        node11.next = node12;
        head2.next = node21;
        node21.next = node22;
        ListNode re = mergeTwoLists(head1,head2);
        while(re!=null) {
            if(re.next == null) {
                System.out.print(re.val);
            }else {
                System.out.print(re.val+"->");
            }
            re = re.next;
        }
    }
}