哈希表 - 链表数组 - C++

Question

我正在尝试通过在链接节点上使用数组（制作链表）来创建哈希表。但是我在哈希表中插入值时遇到困难。当我运行它时，我得到这个：

http://gyazo.com/3a28a70e66b3ea34e08223e5948f49c0.png

这是我的代码：

#include <iostream>
using namespace std;
class Node {
public:
  int num;
  Node * next;
};
class intHashTable {
private:
  int size;
  Node ** table;
public:
  intHashTable(int size);  // construct a new hash table with size elements
  ~intHashTable();     // delete the memory for all internal components
  void insert(int num);    // insert num into the hash table, no effect
               // if num is already in table
  void remove(int num);    // remove num from the hash table, no effect if not in table
  int lookup(int num);     // return 1 if num is already in table, 0 otherwise
  void print(void);        // print the elements of the hash table to the screen
};
// construct a new hash table with nelements elements
intHashTable::intHashTable(int nelements)
{
  size = nelements;
  table = new Node*[size];
  for ( int i = 0; i < size; i++ ) {
    table[i] = NULL;
  }
}
intHashTable::~intHashTable()
{
   for(int i=0; i<size; i++)
   {
      Node* temp = table[i];
      while(temp != NULL)
      {
         Node* next = temp->next;
         delete temp;
         temp = next;
      }
   }
   size = 0;
   delete[] table;
}
void intHashTable::insert(int num){
    int location = ((unsigned)num) % size;
    Node *runner = table[location];
    if(runner == NULL ){
    runner->num = num;
     }else{
        while(runner != NULL ){
            runner = runner->next;
        }
        runner->num = num;
    }
   }
int main(){
    intHashTable a (10);
    a.insert(2);
    return 0;
}

Answer 1

在构造intHashTable之后，table的所有元素仍然NULL。但是，在函数 insert 中，有一个元素被取消引用：

Node *runner = table[location];
runner = runner->next;

这会使程序崩溃，因为取消引用空指针是非法的。

Answer 2

这里的逻辑是错误的

int location = ((unsigned)num) % size;
Node *runner = table[location];
if(runner == NULL ) // if null u dereference it!
{
 runner->num = num;
}
else
{
  while(runner != NULL ) {  // u loop until null
    runner = runner->next;
  }
  runner->num = num;  // once u reach null u dereference it!
}

相反，我建议：

首先是节点的ctor

class Node {
public:
  int num;
  Node * next;
  Node( int _n ) : num(_n), next(NULL) { } 
};

然后

if ( runner != NULL )
{
   while ( runner->next != NULL )
   {
      runner = runner->next;
   }
   runner->next = new Node( num );
}
else
{
  table[location] = new Node( num ); 
}

Answer 3

这段代码肯定行不通：

if(runner == NULL ){
runner->num = num;

如果 runner 为 NULL，则永远不应取消引用它（在其上使用 * 或 ->）。

Answer 4

Node *runner = table[location];
runner = runner->next;
if(runner == NULL )

您从未验证过table[location]是否为空。但是在构建哈希表期间，节点表内没有节点（您将每个条目都设置为 null）。

代码的问题在于你从不考虑分配节点。你应该做

Node* toInsert = new Node;
toInsert->next= NULL;
toInsert->num = num;
if(table[location]==NULL){
   table[location] = toInsert;  
}
else{
    Node *runner = table[location];
    while(runner->next != NULL){
         runner = runner->next;
    }
    runner->next = toInsert;
}