选择专项变式 4：遍历链表求和

#include <stdio.h>

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

int main(void)
{
    struct Node a = {5, NULL}, b = {8, NULL}, c = {6, NULL};
    struct Node *p = &a;
    int sum = 0;
    a.next = &b;
    b.next = &c;
    while (【0】)
    {
        sum += p->data;
        【1】;
    }
    printf("%d\n", sum);
    return 0;
}

程序求链表 5 → 8 → 6 所有结点的和。【0】【1】处应填什么？如果【0】错填成 p->next != NULL，输出会是【2】。