Inserting in a binary search tree access violation error

问题

I am trying to insert node in a binary search tree, I am getting an access voilation error at line if(ptr->data== item) in searchNode() function. How can I remove it. I am new to debugging.

I am first trying to insert few nodes and then display them using display function. During insertion the program searches for the appropriate position of the node to be inserted and then inserts it. Program simply returns if node already exists.

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
struct node
{
       int data;
       struct node* left;
       struct node* right;
};
void searchNode(struct node* root,int item,struct node* loc,struct node *par)
{
              struct node * ptr,*save;
               if(root==NULL)
               {
                             loc=NULL;
                             par=NULL;
                             return;
               }
               else
               {
                   if(item==root->data)
                   {

                                  par=NULL;loc=root;  return;
                   }
                   else if(item < root->data)
                   {
                         save=root;ptr=root->left;
                   }
                   else if(item > root->data)
                   {
                        save=root;ptr=root->right;
                   }               

                   while(ptr!=NULL)
                   {
                           if(ptr->data == item )
                           {
                                              loc=ptr;
                                              par=save;
                                           return;
                           }
                          else if(ptr->data > item )
                          {
                               save=ptr;
                               ptr=ptr->left;        
                          }
                          else
                          {
                              save=ptr;ptr=ptr->right;
                          }
                   }
                   loc=NULL;
                   par=save;
               }
}
void insertNode(struct node* root,int item, struct node * loc)
{
     struct node* par,*newNode;
     searchNode(root,item,loc,par);
     if(loc!=NULL)
               return;
     newNode=(struct node *)malloc(sizeof(struct node));
     newNode->left=NULL;newNode->right=NULL;
     if(par==NULL)
     {
                  root=newNode;
     }
     else if(item< par->data)
     {
          par->left=newNode;
     }
     else if(item> par->data)
     {
          par->right=newNode;
     }
}
void display(struct node* t, int level)
{
     int i;
     if(t)
     {
          display(t->right,level+1);
          printf("\n");
          for(i=0;i<level;i++)
          printf(" ");
          printf("%d",t->data);
          display(t->left,level+1);
     }
}
int main()
{
    int n,data,i;
    struct node* root,*loc;
    scanf("%d",&n);
    for(i=0;i<n;i++)
    {
                    scanf("%d",&data);
                    insertNode(root,data,loc);
    }
    display(root,1);
    getch();
    return 0;
}

回答1:

You are calling your insert method as:

insertNode(root,data,loc);

and the root is passed by value as a result, any changes made to root in the insertNode method will not be visible in main. Since your root in main is uninitialized, the same gets passed to display where you try to dereference the uninitialized pointer leading to undefined behavior.

To fix this you either pass the address of root to insertNode function or return the changed root from the function.

来源：https://stackoverflow.com/questions/13412076/inserting-in-a-binary-search-tree-access-violation-error