19. Remove Nth Node From End of List - Solved in Java/Python/C++/JavaScript/C#/Go/Ruby

LeetCode Python/Java/C++/JS > Linked List > 19. Remove Nth Node From End of List > Solved in Java, Python, C++, JavaScript, C#, Go, Ruby > GitHub or Repost

LeetCode link: 19. Remove Nth Node From End of List, difficulty: Medium.

Given the head of a linked list, remove the n^th node from the end of the list and return its head.

Example 1:

Input: head = [1,2,3,4,5], n = 2

Output: [1,2,3,5]

Example 2:

Input: head = [1], n = 1

Output: []

Example 3:

Input: head = [1,2], n = 1

Output: [1]

Constraints:

The number of nodes in the list is sz.
1 <= sz <= 30
0 <= Node.val <= 100
1 <= n <= sz

Hint 1

Maintain two pointers and update one with a delay of n steps.

Intuition
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Deleting the n^th to last node in the linked list is equivalent to deleting the (node_count - n)^th node in the linked list.
First find out node_count.
When index == node_count - n, delete the node by node.next = node.next.next.
Since the deleted node may be head, a virtual node dummy_node is used to facilitate unified processing.

Step-by-Step Solution

First find out node_count.

node_count = 0
node = head

while node
  node_count += 1
  node = node.next
end

When index == node_count - N, delete the node by node.next = node.next.next.

index = 0
node = head

while node
  if index == node_count - n
    node.next = node.next.next
    break
  end

  index += 1
  node = node.next
end

Since the deleted node may be head, a virtual node dummy_node is used to facilitate unified processing.

dummy_head = ListNode.new # 1
dummy_head.next = head # 2
node = dummy_head # 3

# omitted code

return dummy_head.next

Complexity

Time complexity

O(N)

Space complexity

O(1)

Java #

/**
 * 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 removeNthFromEnd(ListNode head, int n) {
        var nodeCount = 0;
        var node = head;

        while (node != null) {
            nodeCount++;
            node = node.next;
        }

        var index = 0;
        var dummyHead = new ListNode(0, head);
        node = dummyHead;

        while (node != null) {
            if (index == nodeCount - n) {
                node.next = node.next.next;
                break;
            }

            index++;
            node = node.next;
        }

        return dummyHead.next;
    }
}

Python #

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
        node_count = 0
        node = head

        while node:
            node_count += 1
            node = node.next

        index = 0
        dummy_head = ListNode(next=head)
        node = dummy_head

        while node:
            if index == node_count - n:
                node.next = node.next.next
                break

            index += 1
            node = node.next

        return dummy_head.next

C++ #

/**
 * 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* removeNthFromEnd(ListNode* head, int n) {
        auto node_count = 0;
        auto node = head;

        while (node != nullptr) {
            node_count += 1;
            node = node->next;
        }

        auto index = 0;
        auto dummy_head = new ListNode(0, head);
        node = dummy_head;

        for (auto i = 0; i < node_count - n; i++) {
            node = node->next;
        }

        auto target_node = node->next;
        node->next = node->next->next;
        delete target_node;

        auto result = dummy_head->next;
        delete dummy_head;
        return result;
    }
};

JavaScript #

/**
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
var removeNthFromEnd = function (head, n) {
  let nodeCount = 0
  let node = head

  while (node != null) {
    nodeCount++
    node = node.next
  }

  let index = 0
  let dummyHead = new ListNode(0, head)
  node = dummyHead

  while (node != null) {
    if (index == nodeCount - n) {
        node.next = node.next.next
        break
    }

    index++
    node = node.next
  }

  return dummyHead.next
};

C# #

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution
{
    public ListNode RemoveNthFromEnd(ListNode head, int n)
    {
        int nodeCount = 0;
        var node = head;

        while (node != null)
        {
            nodeCount++;
            node = node.next;
        }

        int index = 0;
        var dummyHead = new ListNode(0, head);
        node = dummyHead;

        while (node != null)
        {
            if (index == nodeCount - n)
            {
                node.next = node.next.next;
                break;
            }

            index++;
            node = node.next;
        }

        return dummyHead.next;
    }
}

Go #

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func removeNthFromEnd(head *ListNode, n int) *ListNode {
    nodeCount := 0
    node := head

    for node != nil {
        nodeCount++
        node = node.Next
    }

    index := 0
    dummyHead := &ListNode{0, head}
    node = dummyHead

    for node != nil {
        if index == nodeCount - n {
            node.Next = node.Next.Next
            break
        }

        index++
        node = node.Next
    }

    return dummyHead.Next
}

Ruby #

# class ListNode
#     attr_accessor :val, :next
# 
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end

def remove_nth_from_end(head, n)
  node_count = 0
  node = head

  while node
    node_count += 1
    node = node.next
  end

  index = 0
  dummy_head = ListNode.new(0, head)
  node = dummy_head

  while node
    if index == node_count - n
      node.next = node.next.next
      break
    end

    index += 1
    node = node.next
  end

  dummy_head.next
end

Other languages

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