19. Remove Nth Node From End of List

/**
 * 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) {
        //creating a dummy node
        ListNode* newHead = new ListNode(-1);
        newHead -> next = head;
        ListNode* slow = newHead;
        ListNode* fast = newHead;

        for(int i=0; i<n; i++)
        {
            fast = fast -> next;
        }

        while(fast -> next)
        {
            slow = slow -> next;
            fast = fast -> next;
        }
        //after this slow will point to the just before n + 1 node from back

        slow -> next = slow -> next -> next;

        //first is the dummy node
        return newHead -> next;
    }
};