LeetCode（21）：合并两个有序链表

Dear 丶 2021-10-13 22:56 489阅读 0赞

将两个有序链表合并为一个新的有序链表并返回。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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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;
        }
    }

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public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // maintain an unchanging reference to node ahead of the return node.
        ListNode prehead = new ListNode(-1);
        ListNode prev = prehead;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        // exactly one of l1 and l2 can be non-null at this point, so connect
        // the non-null list to the end of the merged list.
        prev.next = l1 == null ? l2 : l1;
        return prehead.next;
    }