为什么我遇到分段错误

Question

下面的代码是我整个程序的一个片段。我定义的唯一内容是构造函数（head = 0），析构函数和一个旨在创建包含20个值的链表的函数。它们应该是随机的（我可以处理），但我需要先修复这个 seg 错误。帮助！

节点的结构类型为：

struct ListNode
{
    int value;
    ListNode *next;
};
ListNode *head;

法典：

void NumberList::buildSortedList(){
   ListNode *newNode;   
   int number = 1;    
   int count = 0;    
   head->value = number;    
   ListNode *p;   
   do{ 
      p = head;       
      while (p->next) { // traverse the list to the end to append new value 
         p = p->next;     
      newNode = new ListNode; 
      newNode->value = number; 
      newNode->next = NULL;       
      p->next = newNode;      
     }         
   }while (count < 19);   
   }

Answer 1

  while (p->next) { // traverse the list to the end to append new value 
     p = p->next;
  newNode = new ListNode; 
  newNode->value = number; 
  newNode->next = NULL; 
  p->next = newNode; 
 }

应该是

  while (p->next) { // traverse the list to the end to append new value 
     p = p->next;
  }
  newNode = new ListNode; 
  newNode->value = number; 
  newNode->next = NULL; 
  p->next = newNode; 

Answer 2

如果head是全局变量，则初始化为 NULL 。取消引用NULL是非法的。

在取消引用之前，必须将指向有效缓冲区的指针分配给该缓冲区。

void NumberList::buildSortedList(){
   ListNode *newNode;
   int number = 1; 
   int count = 0; 
   if (head == NULL) head = new ListNode; // add this line
   head->value = number; 
   ListNode *p; 
   do{ 
      p = head; 
      while (p->next) { // traverse the list to the end to append new value 
         p = p->next;
      }  // move this in order not to break the list
      newNode = new ListNode; 
      newNode->value = number; 
      newNode->next = NULL; 
      p->next = newNode; 

   }while (count++ < 19); // add ++ to avoid infinite loop
}