【算法题系列之七】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

/**
	 * Definition for singly-linked list.
	 * public class ListNode {
	 *     int val;
	 *     ListNode next;
	 *     ListNode(int x) { val = x; }
	 * }
	 */

    // 方法一：递归方式
	public static ListNode mergeTwoLists1(ListNode l1, ListNode l2) {
		ListNode result = null;
		if (l1 == null) {
			return l2;
		}
		if (l2 == null) {
			return l1;
		}
		if (l1.val <= l2.val) {
			result = l1;
			result.next = mergeTwoLists1(l1.next, l2); // 递归
		} else {
			result = l2;
			result.next = mergeTwoLists1(l1, l2.next);
		}
		return result;
	}

    // 方法二：非递归方式
	public static ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
		if (l1 == null) {
			return l2;
		}
		if (l2 == null) {
			return l1;
		}
		ListNode mergeNode = null;
		if (l1.val <= l2.val) {
			mergeNode = l1;
			l1 = l1.next;
		} else {
			mergeNode = l2;
			l2 = l2.next;
		}
		ListNode temp = mergeNode;
		while (l1 != null && l2 != null) {
			if (l1.val <= l2.val) {
				temp.next = l1;
				l1 = l1.next;
			} else {
				temp.next = l2;
				l2 = l2.next;
			}
			temp = temp.next;
		}
		if (l1 != null) {
			temp.next = l1;
		}
		if (l2 != null) {
			temp.next = l2;
		}
		return mergeNode;
	}