求链表倒数第n个元素

提示：设置一前一后两个指针，一个指针步长为1，另一个指针步长为n，当一个指针走到链表尾端时，

 另一指针指向的元素即为链表倒数第n个元素。

#include <stdio.h>
#include <stdlib.h>
#include "LinkList.h"

void createLinkList(LNode **p) {
	setnull(p); /*建议链表并设置为空表*/
	int n = 10, i;
	srand(time(0));
	for (i = 1; i <= n; i++) {
		insert(p, rand() % 100 + 1, i);
	} /*插入数据到链表*/
}
LNode* getLNodeByIndex(LNode* head, int index) {
	if (!head || index < 1) {
		return NULL;
	}
	int j = 1;
	LNode *q = head;
	while (j < index && q != null) {
		q = q->next;
		j++;
	}
	if (q != null) {
		return q;
	} else {
		printf("位置参数不正确!\n");
		return NULL;
	}
}

LNode* getNumByIndexFromRear(LNode* head, int index) {
	LNode* begin = head;
	LNode* end = getLNodeByIndex(head, index);
	if (!end) {
		return NULL;
	}
	while (end->next) {
		begin = begin->next;
		end = end->next;
	}

	return begin;
}
LNode* findLastElem(LNode* head, int last) {
	if (!head) {
		return NULL;
	}
	LNode* prev = head;
	LNode* next = head;
	while (last-- && next) {
		next = next->next;
	}
	if (last+1) {
		return NULL;
	}
	while (next) {
		prev = prev->next;
		next = next->next;
	}
	return prev;
}

int main(void) {
	setvbuf(stdout, NULL, _IONBF, 0);

	LNode *head; /*定义静态变量*/

	createLinkList(&head);
	display(&head); /*显示链表所有数据*/

	int index = 3;
	puts("input last index:");
	scanf("%d", &index);
//	LNode* node = getNumByIndexFromRear(head, index);
	LNode *node = findLastElem(head, index);
	if (node) {
		printf("LastIndex%3d:, Elem:%3d\n", index, node->data);
	} else {
		puts("Invalid Input.");
	}
	return EXIT_SUCCESS;
}