21. 合并两个有序链表

传统的都是再建一个新链表，但是这样子占用空间太大了
我就想着能否有一个在原来两条链表的基础上进行操作的方法
就写了这个解法，leetcode上的速度内存分析如下：

执行用时 :  2 ms , 在所有 Java 提交中击败了95.47%的用户
内存消耗 :  34.7 MB, 在所有 Java 提交中击败了97.09%的用户

直接上代码：

  public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        ListNode before = null;
        ListNode head = null;
        ListNode another = null;
        ListNode result = null;
        if(l1.val <= l2.val) {
            head = l1;
            result = head;
            before = head;
            another = l2;
            head = head.next;
        }else{
            head = l2;
            result = head;
            before = head;
            another = l1;
            head = head.next;
        }
        //todo: 用head.next 或者another.next 来进行判断即可。
        while(head != null && another != null){
            if(head.val <= another.val){
                before.next = head;
                head = head.next;
                before = before.next;
            }else{
                before.next = another;
                another = another.next;
                before = before.next;
            }
        }
        if(head != null && another == null) {
           before.next = head;
        }
        if(head == null && another != null) {
            before.next = another;
        }
        return result;
    }