[LeetCode] 19. Remove Nth Node From End of List

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

For example,

   Given linked list: 1->2->3->4->5, and n = 2.

   After removing the second node from the end, the linked list becomes 1->2->3->5.

Note:
Given n will always be valid.
Try to do this in one pass.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        ListNode *ret = NULL;
        ListNode *prev = dummy;
        
        ListNode *p1 = head;
        ListNode *p2 = head;
        int counter = 0;
        
        while (p2){
            p2 = p2->next;
            counter++;
            if (counter == n){
                break;
            }
        }
        
        if (counter < n){
            delete dummy;
            return head;
        }
        
        while(p2){
            p1 = p1->next;
            p2 = p2->next;
            prev = prev->next;
        }
        
        prev->next = p1->next;
        
        ret = dummy->next;
        delete dummy;
        return ret;
    }
};