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1474. 删除链表 M 个节点之后的 N 个节点 🔒

题目描述

给定链表 head 和两个整数 m 和 n. 遍历该链表并按照如下方式删除节点:

  • 开始时以头节点作为当前节点.
  • 保留以当前节点开始的前 m 个节点.
  • 删除接下来的 n 个节点.
  • 重复步骤 2 和 3, 直到到达链表结尾.

在删除了指定结点之后, 返回修改过后的链表的头节点.

 

示例 1:

输入: head = [1,2,3,4,5,6,7,8,9,10,11,12,13], m = 2, n = 3
输出: [1,2,6,7,11,12]
解析: 保留前(m = 2)个结点,  也就是以黑色节点表示的从链表头结点开始的结点(1 ->2).
删除接下来的(n = 3)个结点(3 -> 4 -> 5), 在图中以红色结点表示.
继续相同的操作, 直到链表的末尾.
返回删除结点之后的链表的头结点.

示例 2:

输入: head = [1,2,3,4,5,6,7,8,9,10,11], m = 1, n = 3
输出: [1,5,9]
解析: 返回删除结点之后的链表的头结点.

示例 3:

输入: head = [1,2,3,4,5,6,7,8,9,10,11], m = 3, n = 1
输出: [1,2,3,5,6,7,9,10,11]

示例 4:

输入: head = [9,3,7,7,9,10,8,2], m = 1, n = 2
输出: [9,7,8]

 

提示:

  • 链表中节点数目在范围 [1, 104] 内
  • 1 <= Node.val <= 106
  • 1 <= m, n <= 1000

 

进阶: 你能通过 就地 修改链表的方式解决这个问题吗?

解法

方法一：模拟

我们可以模拟整个删除过程，首先用 \(\textit{pre}\) 指针指向链表头部，然后遍历链表，移动 \(m - 1\) 步，如果 \(\textit{pre}\) 为空，说明从当前节点开始的节点个数小于 \(m\)，直接返回头部；否则，用 \(\textit{cur}\) 指针指向 \(\textit{pre}\)，然后移动 \(n\) 步，如果 \(\textit{cur}\) 为空，说明从 \(\textit{pre}\) 开始的节点个数小于 \(m + n\)，直接将 \(\textit{pre}\) 的 \(\textit{next}\) 指向 \(\text{null}\)；否则，将 \(\textit{pre}\) 的 \(\textit{next}\) 指向 \(\textit{cur}\) 的 \(\textit{next}\)，然后将 \(\textit{pre}\) 移动到 \(\textit{pre}\) 的 \(\textit{next}\)。继续遍历链表，直到 \(\textit{pre}\) 为空，返回头部。

时间复杂度 \(O(n)\)，其中 \(n\) 是链表中节点的个数。空间复杂度 \(O(1)\)。

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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def deleteNodes(self, head: ListNode, m: int, n: int) -> ListNode:
        pre = head
        while pre:
            for _ in range(m - 1):
                if pre:
                    pre = pre.next
            if pre is None:
                return head
            cur = pre
            for _ in range(n):
                if cur:
                    cur = cur.next
            pre.next = None if cur is None else cur.next
            pre = pre.next
        return head

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteNodes(ListNode head, int m, int n) {
        ListNode pre = head;
        while (pre != null) {
            for (int i = 0; i < m - 1 && pre != null; ++i) {
                pre = pre.next;
            }
            if (pre == null) {
                return head;
            }
            ListNode cur = pre;
            for (int i = 0; i < n && cur != null; ++i) {
                cur = cur.next;
            }
            pre.next = cur == null ? null : cur.next;
            pre = pre.next;
        }
        return head;
    }
}

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteNodes(ListNode* head, int m, int n) {
        auto pre = head;
        while (pre) {
            for (int i = 0; i < m - 1 && pre; ++i) {
                pre = pre->next;
            }
            if (!pre) {
                return head;
            }
            auto cur = pre;
            for (int i = 0; i < n && cur; ++i) {
                cur = cur->next;
            }
            pre->next = cur ? cur->next : nullptr;
            pre = pre->next;
        }
        return head;
    }
};

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/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func deleteNodes(head *ListNode, m int, n int) *ListNode {
    pre := head
    for pre != nil {
        for i := 0; i < m-1 && pre != nil; i++ {
            pre = pre.Next
        }
        if pre == nil {
            return head
        }
        cur := pre
        for i := 0; i < n && cur != nil; i++ {
            cur = cur.Next
        }
        pre.Next = nil
        if cur != nil {
            pre.Next = cur.Next
        }
        pre = pre.Next
    }
    return head
}

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/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function deleteNodes(head: ListNode | null, m: number, n: number): ListNode | null {
    let pre = head;
    while (pre) {
        for (let i = 0; i < m - 1 && pre; ++i) {
            pre = pre.next;
        }
        if (!pre) {
            break;
        }
        let cur = pre;
        for (let i = 0; i < n && cur; ++i) {
            cur = cur.next;
        }
        pre.next = cur?.next || null;
        pre = pre.next;
    }
    return head;
}

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