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双指针，第一个指针先走N步，要注意可能把head删掉，所以要有一个dummy指针。
/**
* 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 = head;
ListNode * dummy = new ListNode(0);
ListNode * p2 = dummy;
dummy -> next = head;
for(int i = 0; i < n; i++){
p1 = p1->next;
}
while(p1){
p1 = p1->next;
p2 = p2->next;
}
if(p2->next){
p2->next = p2->next->next;
}
else{
p2->next = NULL;
}
return dummy->next;
}
};
