输入两个递增排序的链表，合并这两个链表并使新链表中的节点仍然是递增排序的。
示例1：
输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4限制：
0 <= 链表长度 <= 1000
//leetcode submit region begin(Prohibit modification and deletion)
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode head, curNode;
if (l1 == null && l2 == null) {
return null;
}
if (l1 == null) {
head = l2;
curNode = l2;
l2 = l2.next;
} else if (l2 == null) {
head = l1;
curNode = l1;
l1 = l1.next;
} else {
if (l1.val < l2.val) {
head = l1;
curNode = l1;
l1 = l1.next;
} else {
head = l2;
curNode = l2;
l2 = l2.next;
}
}
while (l1 != null || l2 != null) {
if (l1 == null || (l2 != null && l1.val > l2.val)) {
curNode.next = l2;
curNode = l2;
l2 = l2.next;
} else {
curNode.next = l1;
curNode = l1;
l1 = l1.next;
}
}
return head;
}
}
//leetcode submit region end(Prohibit modification and deletion)
