链表合并

题目：
　　将两个升序链表合并为一个新的升序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。

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解法一：递归

public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (null == list1) return list2;
        if (null == list2) return list1;
        if (list1.val < list2.val) {
            list1.next = mergeTwoLists(list1.next, list2);
            return list1;
        }
        list2.next = mergeTwoLists(list1, list2.next);
        return list2;
    }

测试

public static void main(String[] args) {
        ListNode l1 = ListNode.build(new int[]{1, 2, 4, 6});
        ListNode l2 = ListNode.build(new int[]{1, 3, 4});
        new LC_0021().mergeTwoLists(l1, l2).display();
    }

解法二：for循环+双指针

public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode res = new ListNode();
        ListNode tail = res;
        if (null == list1) return list2;
        if (null == list2) return list1;
        while (null != list1 && null != list2) {
            if (list1.val < list2.val) {
                tail.next = list1;
                list1 = list1.next;
            } else {
                tail.next = list2;
                list2 = list2.next;
            }
            tail = tail.next;
        }
        if (null != list1) tail.next = list1;
        if (null != list2) tail.next = list2;
        return res.next;
    }