Question

1474. Delete N Nodes After M Nodes of a Linked List

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Description

You are given the head of a linked list and two integers m and n.

Traverse the linked list and remove some nodes in the following way:

• Start with the head as the current node.
• Keep the first m nodes starting with the current node.
• Remove the next n nodes
• Keep repeating steps 2 and 3 until you reach the end of the list.

Return _the head of the modified list after removing the mentioned nodes_.

 

Example 1:

Input: head = [1,2,3,4,5,6,7,8,9,10,11,12,13], m = 2, n = 3
Output: [1,2,6,7,11,12]
Explanation: Keep the first (m = 2) nodes starting from the head of the linked List  (1 ->2) show in black nodes.
Delete the next (n = 3) nodes (3 -> 4 -> 5) show in read nodes.
Continue with the same procedure until reaching the tail of the Linked List.
Head of the linked list after removing nodes is returned.

Example 2:

Input: head = [1,2,3,4,5,6,7,8,9,10,11], m = 1, n = 3
Output: [1,5,9]
Explanation: Head of linked list after removing nodes is returned.

 

Constraints:

• The number of nodes in the list is in the range [1, 104].
• 1 <= Node.val <= 106
• 1 <= m, n <= 1000

 

Follow up: Could you solve this problem by modifying the list in-place?

Solutions

Solution 1

# 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

/**
 * 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;
  }
}

/**
 * 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;
  }
};

/**
 * 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
}