求解编写错误：从后往前数，找到第n个元素

运行出来应该是这样的，第一个输的是n，就是从后往前数第几个元素，然后输入的是总共的元素。

//这是我写的，不知道哪有问题
int get_nth_last(int n, struct node *head) {
    struct node *p;
    p = head;
    while (p->next == NULL) {
        int i = p->data;
        return i;
    }
    int i = 0;
    while (p != NULL) {
        p = p->next;
        i++;
    }
    while (i - n > 0) {
        head = head->next;
    }
    int number = head->data;
    return number;

}

//这些代码不用改
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

struct node {
    struct node *next;
    int          data;
};

int get_nth_last(int n, struct node *head);
struct node *strings_to_list(int len, char *strings[]);

// DO NOT CHANGE THIS MAIN FUNCTION

int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s n list-elements\n", argv[0]);
        return 1;
    }
    int n = atoi(argv[1]);
    // create linked list from command line arguments
    struct node *head = strings_to_list(argc - 2, &argv[2]);

    int result = get_nth_last(n, head);
    printf("%d\n", result);

    return 0;
}




// DO NOT CHANGE THIS FUNCTION

// create linked list from array of strings
struct node *strings_to_list(int len, char *strings[]) {
    struct node *head = NULL;
    for (int i = len - 1; i >= 0; i = i - 1) {
        struct node *n = malloc(sizeof (struct node));
        assert(n != NULL);
        n->next = head;
        n->data = atoi(strings[i]);
        head = n;
    }
    return head;
}

注意这段代码：

    while (i - n > 0) {
        head = head->next;
    }

它没有改变i和n，所以会死循环知道head发生段错误
建议作如下更改：

    int num = i-n;
    while (num-- > 0) {
        head = head->next;
    }