我试图通过使用双指针在 C 中实现 bst 插入操作，但在下面的代码中出现分段错误，idk 为什么？

Question

I think the main problem lies with pointer also keeping in mind the restriction that I can't return node type pointer. Please help me find out where am I doing the mistake. I have commented the other operations of this code and trying to solve this one first. So the code has switch statements and the irrelevant functions calls have been added as part of comment for now.

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

typedef struct tree {  //definition of structure
    int info;
    //char *Name;  
    struct tree *left;
    struct tree *right;
} tree;

int main()
{
    int flag;
    int choice1, item;
    char ch;

    int main_menu(int *choice1);
    int insert(tree **root, int item);
    int display(tree *root);
  
    tree *root = NULL;

    while (1) {
        system("clear");

        main_menu(&choice1);//calling function to display original menu list

        switch (choice1) {
          case 1:
            printf("\nEnter number to be inserted:");
            scanf("%d", &item);
            insert(&root, item);
            break;
          case 2:
            //delete();
            break;
          case 3: 
            //search();
            break;
          case 4:
            //printf("\nTerminating code.....");
            display(root);
            //return(1);
            break;
      
          default:
            printf("\nInvalid choice!!");
            break;
        }
        getchar();
        printf("Enter y to continue");
        ch = getchar();
        if (ch != 'y')
            break;
    }
    return (1);
}

int main_menu(int *choice1) {
    printf("\t\tThe main menu of operations are listed below ");//showing menu

    printf("\n1.insert a value.\n2.delete a value.\n3.search a value \n4.display");

    printf("\n\nEnter choice:");
    scanf("%d", &*choice1);//taking choice of user as input
    return (1);
}

tree *getnode(int item) { //function to create node and returning node pointer
    tree *p;
    p = (tree *)malloc(sizeof(tree));
  
    p->right = NULL;
    p->left = NULL;
    p->info = item;
 
    return (p);
}

int insert(tree **root, int item) {
    tree *ptr, *ptr1, *new;
    int flag;
  
    flag = 0;
    new = getnode(item);
    if (*root == NULL) {
        *root = new;
        (*root)->left = NULL;
        (*root)->right = NULL;
    } else {
        ptr = *root;
        while ((ptr != NULL) && (flag == 0)) { 
            ptr1 = ptr;
            if (item < ptr->info) { 
                ptr = ptr->left;
            }
            if (ptr->info == item) {   
                flag = 1;
                printf("\nalready present");
                return (1);
            }
            if (item > ptr->info) {
                ptr = ptr->right;
            }
        }
   
        /*if (ptr == NULL) {
            new = getnode(item);
        }*/
        if (ptr1->info < item) {
            ptr1->right = new;
        } else {
            ptr1->left = new;
        }
    }  
    return (1);
}

Answer 1

The problem is in the while loop descending the tree: you should add else clauses so the next if test is not evaluated with the updated value of ptr :

    while ((ptr != NULL) && (flag == 0)) { 
        ptr1 = ptr;
        if (item < ptr->info) { 
            ptr = ptr->left;
        } else
        if (ptr->info == item) {   
            flag = 1;
            printf("\nalready present");
            return (1);
        } else
        if (item > ptr->info) {
            ptr = ptr->right;
        }
    }

Note that the flag indicator is redundant as you return directly from the while loop body when you set it to 1 .

Note also that the prototypes for the functions called from main should be outside of the body of main so they are visible when the functions are defined and any conflicts can be detected by the compiler.

Here is a modified version:

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

typedef struct tree {
    int info;
    //char *Name;  
    struct tree *left;
    struct tree *right;
} tree;

int main_menu(int *choice1);
int insert(tree **root, int item);
int display(tree *root);

// read and discard the rest of the input line
// return EOF at end of file
int flush_input(void) {
    int ch;
    while ((ch = getchar()) != EOF && ch != '\n')
        continue;
    return ch;
}
  
int main() {
    int flag, choice1, item, ch;
    tree *root = NULL;

    for (;;) {
        system("clear");

        main_menu(&choice1);  //calling function to display original menu list

        switch (choice1) {
          case 1:
            printf("\nEnter number to be inserted: ");
            if (scanf("%d", &item) != 1) {
                flush_input();
                break;
            }
            flush_input();
            insert(&root, item);
            break;
          case 2:
            //delete();
            break;
          case 3: 
            //search();
            break;
          case 4:
            //printf("\nTerminating code.....");
            display(root);
            //return(1);
            break;
          default:
            printf("\nInvalid choice!!");
            break;
        }
        printf("Enter y to continue: ");
        /* read and discard the rest of the input line */
        ch = getchar();
        flush_input()
        if (ch != 'y')
            break;
    }
    return 0;
}

int main_menu(int *choice1) {
    printf("\t\tThe main menu of operations are listed below ");//showing menu
    printf("\n1.insert a value.\n2.delete a value.\n3.search a value \n4.display");
    printf("\n\nEnter choice:");
    *choice1 = -1;        //default input incase of scanf failure
    scanf("%d", choice1); //taking choice of user as input
    flush_input();        // discard rest of the line
    return 1;
}

tree *getnode(int item) { //function to create node and returning node pointer
    tree *p = (tree *)malloc(sizeof(tree));
  
    p->right = NULL;
    p->left = NULL;
    p->info = item;
 
    return p;
}

int insert(tree **root, int item) {
    tree *new, *ptr, *ptr1;

    ptr1 = ptr = *root;
    while (ptr != NULL) { 
        ptr1 = ptr;
        if (item < ptr->info) { 
            ptr = ptr->left;
        } else
        if (item > ptr->info) {
            ptr = ptr->right;
        } else {   
            printf("\nalready present\n");
            return 1;
        }
    }
    new = getnode(item);
    if (ptr1 == NULL) {
        *root = new;
    } else
    if (ptr1->info < item) {
        ptr1->right = new;
    } else {
        ptr1->left = new;
    }
    return 1;
}