8_21. 合并两个有序链表

/**递归
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        if(l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next,l2);
            return l1;
        }else{
            l2.next = mergeTwoLists(l1,l2.next);
            return l2;
        }
    }
}

/**哨兵节点
 * 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 = new ListNode(0);
        ListNode temp = head;
        while(l1 != null && l2 != null){
            if(l1.val < l2.val){
                temp.next = new ListNode(l1.val);
                temp = temp.next;
                l1 = l1.next;
            }
            else{
                temp.next = new ListNode(l2.val);
                temp = temp.next;
                l2 = l2.next;
            }         
        }
        while(l1 != null){
            temp.next = l1;
            temp = temp.next;
            l1 = l1.next;
        }
        while(l2 != null){
            temp.next = l2;
            temp = temp.next;
            l2 = l2.next;
        }
        return head.next;
        
    }
}