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

    Given the head of a linked list, remove the nth 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
    PreviousNext

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

    Steps

    1. First find out node_count.

      node_count = 0
node = head

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

    2. 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

    3. 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)

    Solution in languages: Java Python C++ JavaScript C# Go Ruby

    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
    Solution in languages: Java Python C++ JavaScript C# Go Ruby

    Other languages

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