added leetcode solution(19) remove Nth node with code and algortihm

dsa-solutions/lc-solutions/0000-0099/0019.Remove-N-Node-From-end-of-list.md

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+---
+id: remove-nth-node-from-end-of-list
+title: Remove nth node from end of list (LeetCode)
+sidebar_label: 0019-remove-nth-node-from-end-of-list
+tags:
+  - Two Pointers
+  - Linked List
+description: "Given the head of a linked list, remove the nth node from the end of the list and return its head."
+---
+## Problem Description
+
+| Problem Statement                                                                                               | Solution Link                                                                                                             | LeetCode Profile                                        |
+| :-------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------------------ | :------------------------------------------------------ |
+| [Remove nth node from end of list](https://leetcode.com/problems/remove-nth-node-from-end-of-list/description/) | [Remove nth node from end of list on LeetCode](https://leetcode.com/problems/remove-nth-node-from-end-of-list/solutions/) | [Areetra Halder](https://leetcode.com/u/areetrahalder/) |
+### Problem Description
+
+Given the head of a linked list, remove the nth node from the end of the list and return its head.
+
+### Examples
+
+#### 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.
+- $1 <= sz <= 30$
+- $0 <= Node.val <= 100$
+- $1 <= n <= sz$
+### Approach
+A challenging point of this question is that Linked List doesn't have index number, so we don't know which node is the last Nth node from the last.
+
+My strategy is to create dummy pointer and create distance dummy pointer and head pointer.
+### Solution Code
+
+#### Python
+
+```python
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        res = ListNode(0, head)
+        dummy = res
+
+        for _ in range(n):
+            head = head.next
+
+        while head:
+            head = head.next
+            dummy = dummy.next
+
+        dummy.next = dummy.next.next
+
+        return res.next
+```
+#### Java
+
+```java
+class Solution {
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        ListNode res = new ListNode(0, head);
+        ListNode dummy = res;
+
+        for (int i = 0; i < n; i++) {
+            head = head.next;
+        }
+
+        while (head != null) {
+            head = head.next;
+            dummy = dummy.next;
+        }
+
+        dummy.next = dummy.next.next;
+
+        return res.next;        
+    }
+}
+```
+#### C++
+
+```cpp
+class Solution {
+public:
+    ListNode* removeNthFromEnd(ListNode* head, int n) {
+        ListNode* res = new ListNode(0, head);
+        ListNode* dummy = res;
+
+        for (int i = 0; i < n; i++) {
+            head = head->next;
+        }
+
+        while (head != nullptr) {
+            head = head->next;
+            dummy = dummy->next;
+        }
+
+        dummy->next = dummy->next->next;
+
+        return res->next;        
+    }
+};
+```
+### Javascript
+```javascript
+var removeNthFromEnd = function(head, n) {
+    let res = new ListNode(0, head);
+    let dummy = res;
+
+    for (let i = 0; i < n; i++) {
+        head = head.next;
+    }
+
+    while (head) {
+        head = head.next;
+        dummy = dummy.next;
+    }
+
+    dummy.next = dummy.next.next;
+
+    return res.next;    
+};
+```
+## Step by Step Algorithm
+1: Initialize variables:
+
+   - We create a dummy node res with a value of 0 and set its next pointer to the head of the original list. This dummy node helps in handling edge cases when removing the first node.
+   - We initialize another pointer dummy to the dummy node res. This pointer will be used to traverse the list.
+```
+res = ListNode(0, head)
+dummy = res
+```
+2: Move head pointer forward by n nodes:
+
+   - We iterate n times using a for loop to advance the head pointer n nodes forward. This effectively moves head to the nth node from the beginning.
+```
+for _ in range(n):
+    head = head.next
+```
+3: Find the node before the node to be removed:
+
+  - We use a while loop to traverse the list with both head and dummy pointers.
+  - As long as head is not None, we move both head and dummy pointers one node forward in each iteration.
+  - After this loop, dummy will be pointing to the node right before the node to be removed.
+```
+while head:
+    head = head.next
+    dummy = dummy.next
+```
+4: Remove the nth node from the end:
+
+  - Once the loop finishes, dummy will be pointing to the node right before the node to be removed.
+  - We update the next pointer of the node pointed by dummy to skip the next node, effectively removing the nth node from the end.
+```
+dummy.next = dummy.next.next
+```
+5: Return the modified list:
+
+  - Finally, we return the next node after the dummy node res, which is the head of the modified list.
+```
+return res.next
+```
+This algorithm effectively removes the nth node from the end of the linked list by traversing it only once.