19. Remove Nth Node From End of List

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19. Remove Nth Node From End of List

Description

Difficulty: Medium

Related Topics: Linked List, Two Pointers

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

Example 1:

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

Example 2:

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

Example 3:

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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

Follow up: Could you do this in one pass?

Hints/Notes

  • 2023/08/01
  • Use 2 pointers to get the Nth from the end first, helper function
  • Use dummy to avoid the nullptr corner case
  • 0x3F’s solution(checked)

Solution

Language: C++

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/**
 * 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();
        dummy->next = head;
        ListNode *nth = findFromEnd(dummy, n + 1);
        nth->next = nth->next->next;
        return dummy->next;
    }

    ListNode* findFromEnd(ListNode* head, int n) {
        ListNode *p1 = head, *p2 = head;
        for (int i = 0; i < n; i++) {
            p1 = p1->next;
        }
        while (p1) {
            p1 = p1->next;
            p2 = p2->next;
        }
        return p2;
    }
};