preorder traversal using stack - data-structures

Question

i'm trying to implement binary tree, pre order traversal using stack.
here its popping the last left node and after that root=root->right doesnt seem to work. Pls help. here 7 is being popped out and is being displayed and after the the program is ending.
all the functions are working yet not the desired output
#include<stdio.h>
#include<stdlib.h>
#define maxsize 100
int a[maxsize];
int top=-1;
struct node{
int data;
struct node *left, *right;
};
struct node *newNode(int data)
{
struct node *nn;
nn=(struct node *)malloc(sizeof(struct node));
if(!nn)
return;
nn->data=data;
nn->left=nn->right=NULL;
return nn;
};
void push(struct node *root)
{
printf("pushcalled\n");
if(top!=maxsize-1)
a[++top]=root;
}
int isempty()
{
return(top==-1);
}
struct node *pop()
{
printf("popcalled\n");
if(top!=-1)
{
return a[top];
top--;
}
}
void deleteStack()
{
free(a[top--]);
}
void preorder(struct node *root)
{
while(1)
{
while(root)
{
printf("%d\t",root->data);
push(root);
root=root->left;
}
printf("hello\n");
if(isempty())
break;
printf("hello\n");
root=pop();
printf("Popped data is:%d\n",root->data);
root=root->right;
printf("right data is:%d\n",root->data);
}
deleteStack();
}
int main()
{
int data;
struct node *root=newNode(10);
root->left = newNode(11);
root->left->left = newNode(7);
root->right = newNode(9);
root->right->left = newNode(15);
root->right->right = newNode(8);
preorder(root);
return 0;
}

Answer 1

Your logic is correct but there are some errors in your code.
root=root->right;
printf("right data is:%d\n",root->data);
you should check whether the root is null or not before you try to access root->data. That is why you are getting a segmentation fault. So put a condition if(root!=NULL) above printf() statement.
Another mistake is in the implementation of stack's pop.
struct node *pop()
{
printf("popcalled\n");
if(top!=-1)
{
return a[top];
top--;
}
}
it should be like this,
struct node *pop()
{
printf("popcalled\n");
if(top!=-1)
{
struct node* temp = a[top];
top--;
return temp;
}
}
In your code, when you return a[top]; the line below it i.e. top--; never executes and the value of top remains same.