19.Remove Nth Node From End of List(List; Two-Pointers)

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.

思路：双指针，相差n

/**
 * 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* pFirst = head, *pSecond = head;
        for(int i = 0; i < n; i++){
            pFirst = pFirst->next;
        }
        if(pFirst == NULL){
            return head->next;
        }
        while(pFirst->next){
            pFirst = pFirst->next;
            pSecond = pSecond->next;
        }
        pSecond->next = pSecond->next->next;
        return head;
    }
};