Lc21_合并两个有序链表

package homework;

import homework.Lc19.ListNode;

/**
 * 21. 合并两个有序链表 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
 *
 */
public class Lc21 {
    public static class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
        }
    }

    public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode prehead = new ListNode(-1);

        ListNode prev = prehead;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }

        // exactly one of l1 and l2 can be non-null at this point, so connect
        // the non-null list to the end of the merged list.
        prev.next = l1 == null ? l2 : l1;

        return prehead.next;
    }

    public static void main(String[] args) {
        ListNode l1 = new ListNode(1);
        ListNode l2 = new ListNode(2);
        l1.next = l2;
        ListNode l3 = new ListNode(1);
        ListNode l4 = new ListNode(2);
        l3.next = l4;
        mergeTwoLists(l1, l3);
    }
}