19. Remove Nth Node From End of List

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

For example,

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>  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.

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/**
 * 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 *p1,*p2,*pre;
        p1=head;
        p2=head;
        pre=head;
        for(int i = 0; i < n-1 ;i++)
        {
            p1 = p1->next;
        }
        while(p1->next!= nullptr)
        {
            p1 = p1->next;
            pre = p2;
            p2 = p2->next;
        }

        if (p2 == head)
        {
            head = head->next;
        }
        pre->next = p2->next;

        return head;
    }
};