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.

Runtime: 9 ms

class Solution{public:ListNode *removeNthFromEnd(ListNode *head, int n){if (head == NULL)return NULL;ListNode*p1 = head;ListNode*p2 = head;ListNode*pre = NULL;while (n-1 != 0){p1 = p1->next;n--;}while (p1->next){pre = p2;p2 = p2->next;p1 = p1->next;}if (p2 == head)head = head->next;else{pre->next = p2->next;delete p2;}return head;}};