2.2

Implement an algorithm to find the nth to last element of a singly linked list.

分析：查找鏈表倒數第n個元素，這在劍指offer上有相同的習題

Node* get_lastn(Node* head,int n){
	if(head==NULL||n<=0) return NULL;
	Node* front=head;
	Node* behind=head;
	while(n>1){
		behind=behind->next;
		if(behind==NULL) return NULL;
		n--;
	}
	while(behind->next!=NULL){
		front=front->next;
		behind=behind->next;
	}
	return front;
}