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
解法一：
迭代求解。
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
//一定要先实例化一个结点
ListNode head = new ListNode(0);
//定义一个结点变量指向头结点
ListNode l3 = head;
while (l1 != null && l2 != null) {
if (l1.val <= l2.val) {
head.next = l1;
l1 = l1.next;
head = head.next;
} else {
head.next = l2;
l2 = l2.next;
head = head.next;
}
}
while (l1 != null) {
head.next = l1;
l1 = l1.next;
head = head.next;
}
while (l2 != null) {
head.next = l2;
l2 = l2.next;
head = head.next;
}
return l3.next;
}
解法二：
递归求解。
public ListNode mergeTwoLists(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;
}
