LeetCode-21.Merge Two Sorted Lists | 合并两个有序链表

题目

LeetCode
LeetCode-cn

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

Example 1:

Input: l1 = [1,2,4], l2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: l1 = [], l2 = []
Output: []

Example 3:

Input: l1 = [], l2 = [0]
Output: [0]
 

Constraints:
The number of nodes in both lists is in the range [0, 50].
-100 <= Node.val <= 100
Both l1 and l2 are sorted in non-decreasing order.

题解

这道题是经典的考察链表的题目。

解法一：递归

• 终止条件：两条链表分别名为 l1 和 l2，当 l1 为空或 l2 为空时结束
• 返回值：每一层调用都返回排序好的链表头
• 本级递归内容：如果 l1 的 val 值更小，则将 l1.next 与排序好的链表头相接，l2 同理
• O(m+n)O(m+n)，mm 为 l1的长度，nn 为 l2 的长度

//Go
/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func mergeTwoLists(l1 *ListNode, l2 *ListNode) *ListNode {
    if l1 == nil {
        return l2;
    }else if l2 == nil {
        return l1;
    }else if (l1.Val < l2.Val) {
        l1.Next = mergeTwoLists(l1.Next, l2);
        return l1;
    }else {
        l2.Next = mergeTwoLists(l1, l2.Next);
        return l2;
    }
}

执行结果：

leetcode-cn：
执行用时：0 ms, 在所有 Go 提交中击败了100.00%的用户
内存消耗：2.6 MB, 在所有 Go 提交中击败了26.43%的用户

leetcode：
Runtime: 0 ms, faster than 100.00% of Go online submissions for Merge Two Sorted Lists.
Memory Usage: 2.6 MB, less than 51.09% of Go online submissions for Merge Two Sorted Lists.

可以看到，该解法执行用时为0ms，非常高效。

链接

力扣-画解算法：21. 合并两个有序链表