[leetcode 21] Merge Two Sorted Lists

 1 题目：

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.

2 思路：

这个就是归并排序的重新排好两个已经排好序的那个步骤。

好吧，这是第一个一次accped的题目。。

 

3 代码：

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null){
            return l2;
        }
        if(l2 == null){
            return l1;
        }
        ListNode head = new ListNode(0);
        ListNode node = head;
        
        ListNode list1 = l1;
        ListNode list2 = l2;
    
        while(list1 != null && list2 != null){
            if(list1.val > list2.val){
                node.next = list2;
                list2 = list2.next;
            }else{
                node.next = list1;
                list1 = list1.next;
            }
            node = node.next;
        }
        
        /* connect the longer list */
        while(list1 != null){
            node.next = list1;
            list1 = list1.next;
            node = node.next;
        }
        while(list2 != null){
            node.next = list2;
            list2 = list2.next;
            node = node.next;
        }
        
        return head.next;
    }

 看了https://leetcode.com/discuss/16346/java-solution-for-reference 的代码，发现最后两个只要接上链表就行了，不用再遍历一遍了。

也就是：

        if (l1 != null) {
            node.next = list1;
        }
        if (l2 != null) {
            node.next = list2;
        }