C中的二叉搜索树SEGFAULT

Question

I'm implementing BTS in C. I have implement the basic things like searching and inserting. But I have problem with finding the smallest element and largest element. Here is my code:

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

//Begin
typedef struct tree{
    int data;
    struct tree *left;
    struct tree *right;
}tree;
//----------------------------------------------------------------------------------------
tree *insert(tree *root, int data);
tree *newnode(int data);
int search(tree *root, int data);
int findMin(tree *root);
int findMax(tree *root);
//-----------------------------------------------------------------------------------------
int main(void){
    tree *root = malloc(sizeof(root));
    root->left = NULL;
    root->right = NULL;
    insert(root, 15);
    insert(root, 10);
    insert(root, 20);
    printf("%i\n", search(root ,15));
    printf("%i\n", search(root ,20));
    printf("%i\n", search(root ,10));
    printf("%i\n", search(root ,11));
    printf("%i\n", search(root ,17));   
    printf("%i\n", search(root ,10));
    printf("Min: %i\n", findMin(root));
    printf("Max: %i\n", findMax(root));
    return 0;
}
//-----------------------------------------------------------------------------------------
tree *insert(tree *root, int data){
    if(root == NULL){
        root = newnode(data);
    }

    else if(root->data < data)
            root->right = insert(root->right,data);
    else if(root->data > data)
            root->left = insert(root->left,data);
    else{
        perror("the elements already exist!");
    }
    return root;
}
//-----------------------------------------------------------------------------------------
tree *newnode(int data){
    tree *new = malloc(sizeof(tree));
    new->data = data;
    new->left = NULL;
    new->right = NULL;
    return new;
}
//-----------------------------------------------------------------------------------------
int search(tree *root, int data){
    if(root == NULL){
        return 0;
    }
    else if(root->data == data){
        return root->data;
    }
    else if (root->data < data){
        return search(root->right,data);
    }
    else if (root->data > data){
        return search(root->left,data);
    }
    else{
        perror("Error");
    }
}

//-----------------------------------------------------------------------------------------
int findMin(tree *root){
    tree *temp = root;
    while(temp != NULL){
        temp = temp->left;
    }
    return temp->data;
}
//-----------------------------------------------------------------------------------------
int findMax(tree *root){
    tree *temp = root;
    while(temp != NULL){
        temp = temp->right;
    }
    return temp->data;
}
//End 

The fault is here: 81 return temp->data;

Ie the while loop in the findmin function

Answer 1

You are dereferencing NULL pointer in both findMin() and findMax() functions.

int findMin(tree *root){
 while(temp != NULL){
        temp = temp->right;
    }
 return temp->data; <-- problem here
} 

The while loop exits when temp becomes NULL. But you are dereferencing the temp after it becomes NULL .

Same problem with findMax() too. You want to return the last node's value before it becomes NULL. You can modify your functions to:

int findMin(tree *root){
    int min;
    tree *temp = root;
    while(temp != NULL){
        min = temp->data;
        temp = temp->left;
    }
    return min;
}

int findMax(tree *root){
    int max;
    tree *temp = root;
    while(temp != NULL){
        max=temp->data;
        temp = temp->right;
    }
    return max;
}

so as to return the min and max values.

Answer 2

You are not returning temp->data until you have tested that temp is null; (null)->data will get you a segmentation fault.

Try fixing your loops like this:

int findMin(tree *root){
   tree *temp = root;
   while(temp -> left != NULL){
      temp = temp->left;
   }
   return temp->data;
}
int findMax(tree *root){
    tree *temp = root;
    while(temp->right != NULL){
        temp = temp->right;
    }
    return temp->data;
}