# LC 19. Remove Nth Node From End of List

### [Problem link](https://leetcode.com/problems/remove-nth-node-from-end-of-list/)

###### tags: `leedcode` `medium` `python` `c++` `Linked List`

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

**Example 1:**
<img alt="" src="https://assets.leetcode.com/uploads/2020/10/03/remove_ex1.jpg" style="width: 542px; height: 222px;" />

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 <code>sz</code>.
- <code>1 <= sz <= 30</code>
- <code>0 <= Node.val <= 100</code>
- <code>1 <= n <= sz</code>

**Follow up:** Could you do this in one pass?
## Solution 1 - Linked List(Follow up)
#### 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]:
        dummy = ListNode(next = head)
        fast = slow = dummy

        for _ in range(n):
            fast = fast.next

        while fast.next:
            fast = fast.next
            slow = slow.next

        slow.next = slow.next.next

        return dummy.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) {
        ListNode *dummy = new ListNode(0, head);
        ListNode *fast = dummy;
        for (int i = 0; i < n; i++) {
            fast = fast->next;
        }
        ListNode *slow = dummy;
        while (fast->next != nullptr) {
            fast = fast->next;
            slow = slow->next;
        }
        slow->next = slow->next->next;
        return dummy->next;
    }
};

>### Complexity
>n = The number of nodes in the list.
>| | Time Complexity | Space Complexity |
>| ------------------- | --------------- | ---------------- |
>| Solution 1(C++) | O(n) | O(1) |
>| Solution 1(Python) | O(n) | O(n) |

## Note
建議紙筆畫一畫.