合併兩有序鏈表 Merge Two Sorted Lists

問題：

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

解決：

①遞歸方法，耗時14ms.

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

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if(l2 == null) return l1;
        if (l1.val < l2.val) {
            ListNode tmp = l1;
            tmp.next = mergeTwoLists(l1.next,l2);            
        }else{
            ListNode tmp = l2;
            tmp.next = mergeTwoLists(l1,l2.next);
        }
        return tmp;
    }
}

②非遞歸方法，新建一個頭節點，依次比較兩個鏈表的值，按照順序添加到頭節點後面便可，耗時16ms。

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode header = new ListNode(-1);
        ListNode cur = header;
        while(l1 != null && l2 != null){
            if (l1.val < l2.val) {
                cur.next = l1;
                l1 = l1.next;
            }else{
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        cur.next = (l1 != null) ? l1 : l2;
        return header.next;
    }
}