使用父指针从二进制搜索树中删除节点

Question

I am working on an algorithm to delete a node with a given key from a binary search tree. So far, I have been able to come up with the following code:

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <time.h>

typedef int ElType;

typedef struct Tree {
    ElType key;
    struct Tree *left;
    struct Tree *right;
    struct Tree *parent;
} Tree;

Tree* InsertBST(Tree* t, int k)
{
    if (t == NULL) {
        Tree* w = (Tree*) malloc(sizeof(Tree));
        w->key = k;
        w->left = NULL;
        w->right = NULL;
        w->parent = NULL;
        return w;
    }

    if (k <= t->key) {
        t->left = InsertBST(t->left, k);
        t->left->parent = t;
    }
    else {
        t->right = InsertBST(t->right, k);
        t->right->parent = t;
    }

    return t;
}

Tree* DeleteMaxOfBST(Tree* t, ElType *deleted_value)
{
    if (t == NULL) {
        *deleted_value = -1;
        return NULL;
    }

    if (t->right == NULL) {
        *deleted_value = t->key;
        Tree* w = t->left;
        w->parent = t->parent;
        free(t);
        return w;
    }

    t->right = DeleteMaxOfBST(t->right, deleted_value);
    return t;
}

Tree* DeleteNodeOfBST(Tree* t, int k)
{
    if (t == NULL) return NULL;

    if (k < t->key) {
        t->left = DeleteNodeOfBST(t->left, k);
        return t;
    }
    else if (k > t->key) {
        t->right = DeleteNodeOfBST(t->right, k);
        return t;
    }
    else if (t->left == NULL) {
        Tree* w = t->right;
        w->parent = t->parent;
        free(t);
        return w;
    }
    else {
        ElType max_left;
        t->left = DeleteMaxOfBST(t->left, &max_left);
        t->key = max_left;
        return t;
    }
}

The general idea is that I want to work with a BST with pointers to parent nodes and be able to delete a node with whichever key I specify while preserving the structure of a BST.

My code works for some keys in some trees but crashes for other keys without any apparent pattern. I then get the following error:

Segmentation fault (core dumped)

I am inclined to think I have messed up the pointers to the parent nodes but cannot quite pinpoint where the fault is. I am relatively new to C, so I would appreciate any comments whether pointers are in fact the problem here and how to possibly fix this.

Answer 1

So, without any examples of how your code runs it's hard to say where exactly the segmentation fault is occurring when your program is running. When your program encounters a segmentation fault that means that the program is attempting to access memory that, for whatever reason, it is unable to. This generally means your pointers are trying to point at an address in memory that they shouldn't be.

My suggestion would be to run the code step by step and see where the problem occurs. Or find a debugger that can show you the memory issues your program is having. I know that the program Valgrind exists for Ubuntu and other Linux best machines, but I'm not sure what others are available for other OSes. You can read more about Valgrind here: http://valgrind.org/ . I use it whenever I need to check for potential memory handling issues in my programs.

Other than that, just keep a real close eye on the space that you create using malloc, as well as where your pointers are pointing. Make sure to reconnect your tree properly when you delete the given node. Manually handling memory can be a pain, but you'll get the hang of it.

Answer 2

Here is the source code for C Program for Insertion and Deletion in Binary Search Tree Recursive..................

 /*  C Program for Insertion and Deletion in Binary Search Tree Recursive  */

#include<stdio.h>
#include<stdlib.h>

struct node
{
    struct node *lchild;
    int info;
    struct node *rchild;
};

struct node *insert(struct node *ptr, int ikey);
void display(struct node *ptr,int level);
struct node *del(struct node *ptr, int dkey);

int main( )
{
    struct node *root=NULL,*ptr;
    int choice,k;
    while(1)
    {
        printf("\n");
        printf("1.Insert\n");
        printf("2.Delete\n");
        printf("3.Display\n");
        printf("4.Quit\n");
        printf("\nEnter your choice : ");
        scanf("%d",&choice);

        switch(choice)
        {
        case 1:
            printf("Enter the key to be inserted : ");
            scanf("%d",&k);
            root = insert(root, k);
            break;
        case 2:
            printf("Enter the key to be deleted : ");
            scanf("%d",&k);
            root = del(root,k);
            break;
         case 3:
             display(root,0);
             break;
         case 4:
            exit(1);
         default:
            printf("\nWrong choice\n");
        }/*End of switch */
    }/*End of while */

    return 0;

}/*End of main( )*/

struct node *insert(struct node *ptr, int ikey )
{
    if(ptr==NULL)
    {
        ptr = (struct node *) malloc(sizeof(struct node));
        ptr->info = ikey;
        ptr->lchild = NULL;
        ptr->rchild = NULL;
    }
    else if(ikey < ptr->info)   /*Insertion in left subtree*/
        ptr->lchild = insert(ptr->lchild, ikey);
    else if(ikey > ptr->info)   /*Insertion in right subtree */
        ptr->rchild = insert(ptr->rchild, ikey);
    else
        printf("\nDuplicate key\n");
    return ptr;
}/*End of insert( )*/

void display(struct node *ptr,int level)
{
    int i;
    if(ptr == NULL )/*Base Case*/
        return;
    else
    {
        display(ptr->rchild, level+1);
        printf("\n");
        for (i=0; i<level; i++)
            printf("    ");
        printf("%d", ptr->info);
        display(ptr->lchild, level+1);
    }

       printf("\n");

}/*End of display()*/


struct node *del(struct node *ptr, int dkey)
{
    struct node *tmp, *succ;

    if( ptr == NULL)
    {
        printf("dkey not found\n");
        return(ptr);
    }
    if( dkey < ptr->info )/*delete from left subtree*/
        ptr->lchild = del(ptr->lchild, dkey);
    else if( dkey > ptr->info )/*delete from right subtree*/
        ptr->rchild = del(ptr->rchild, dkey);
    else
    {
        /*key to be deleted is found*/
        if( ptr->lchild!=NULL  &&  ptr->rchild!=NULL )  /*2 children*/
        {
            succ=ptr->rchild;
            while(succ->lchild)
                succ=succ->lchild;
            ptr->info=succ->info;
            ptr->rchild = del(ptr->rchild, succ->info);
        }
        else
        {
            tmp = ptr;
            if( ptr->lchild != NULL ) /*only left child*/
                ptr = ptr->lchild;
            else if( ptr->rchild != NULL) /*only right child*/
                ptr = ptr->rchild;
            else    /* no child */
                ptr = NULL;
            free(tmp);
        }
    }
    return ptr;
}/*End of del( )*/

Hope it may help you. For more details, Visit here for More Operations on Binary Search tree ---> C Program for Insertion and Deletion in Binary Search Tree Recursive

and C Program for binary search tree deletion without recursion