Hi,

I need help on my program. The program is to write an iterative version (using stack )of a postorder traversal on a binary tree.

The function for inserting tree works perfectly but when i call on the function iter_postorder()...nothing happens. Please tell me where I went wrong.

#define CompLT(a,b) (a<b)

#define CompEQ(a,b) (a==b)

#define MAX_STACK_SIZE 100

typedef struct

{

char name [5];

struct Node *link;

} tree;

tree tree1;

typedef struct Node

{

struct Node *left;

struct Node *right;

struct Node *parent;

int data;

}node;

void iter_postorder(node *a)

{

int top=-1;

node *stack[MAX_STACK_SIZE];

for(;;)

{

for(;a;a=a->left)

{

if(top>=MAX_STACK_SIZE-1)

{

fprintf(stderr,"\nStack is full\n");

return;

}

stack[++top]=a;

}

if (top==-1)

{

fprintf(stderr,"\nStack is empty\n");

return;

}

a=stack[top--];

if(!a->right)

{

printf("%d ",a->data);

a=NULL;

}

else

a=a->right;

}

}

void tread (tree *a)

{

int key;

node *x, *current, *parent;

printf("\nWhat is the name of this tree? ");

fgets(a->name,5,stdin);

printf("\nInput the keys of this tree (-1 to end): \n");

scanf("%d",&key);

while (key!=-1)

{

x=(node*)malloc(sizeof(node));

x->data=key;

x->left=NULL;

x->right=NULL;

current=a->link;

parent=0;

while(current)

{

parent=current;

current=CompLT(x->data,current->data) ? current->left:current->right;

}

if(parent)

{

if(CompLT(x->data,parent->data))

parent->left=x;

else

parent->right=x;

}

else

a->link=x;

scanf("%d",&key);

}

}

int main(void)

{ /*calling functions*/

tread(&tree1);

iter_postorder(tree1.link);

}

I need help on my program. The program is to write an iterative version (using stack )of a postorder traversal on a binary tree.

The function for inserting tree works perfectly but when i call on the function iter_postorder()...nothing

#define CompLT(a,b) (a<b)

#define CompEQ(a,b) (a==b)

#define MAX_STACK_SIZE 100

typedef struct

{

char name [5];

struct Node *link;

} tree;

tree tree1;

typedef struct Node

{

struct Node *left;

struct Node *right;

struct Node *parent;

int data;

}node;

void iter_postorder(node *a)

{

int top=-1;

node *stack[MAX_STACK_SIZE];

for(;;)

{

for(;a;a=a->left)

{

if(top>=MAX_STACK_SIZE-1)

{

fprintf(stderr,"\nStack is full\n");

return;

}

stack[++top]=a;

}

if (top==-1)

{

fprintf(stderr,"\nStack is empty\n");

return;

}

a=stack[top--];

if(!a->right)

{

printf("%d ",a->data);

a=NULL;

}

else

a=a->right;

}

}

void tread (tree *a)

{

int key;

node *x, *current, *parent;

printf("\nWhat is the name of this tree? ");

fgets(a->name,5,stdin);

printf("\nInput the keys of this tree (-1 to end): \n");

scanf("%d",&key);

while (key!=-1)

{

x=(node*)malloc(sizeof(nod

x->data=key;

x->left=NULL;

x->right=NULL;

current=a->link;

parent=0;

while(current)

{

parent=current;

current=CompLT(x->data,cur

}

if(parent)

{

if(CompLT(x->data,parent->

parent->left=x;

else

parent->right=x;

}

else

a->link=x;

scanf("%d",&key);

}

}

int main(void)

{ /*calling functions*/

tread(&tree1);

iter_postorder(tree1.link)

}

This one is on us!

(Get your first solution completely free - no credit card required)

UNLOCK SOLUTION
The other observation here is that only the leaf nodes get displayed on the screen. The reason is simple - all the non-leaf nodes get popped from the stack but do not get displayed. The solution is to push them back onto the stack. This brings us to one more question - if they are pushed back onto the stack, then the next time they are popped they will still have a right node & hence it will translate into an infinite loop!

This infinite loop can be avoided by checking if the popped out node's right child has already been processed. So when you are printing the popped out node, check if this node is the right child of the next node on the stack. If yes the next node has been completely processed & hence needs to be popped & printed.

BlackBeauty, note that I arrived at the following solution based on your code. My objective was to work on your code not to rewrite it. Hence this may not be the most elegant solution but it works

Also I apologize if my explanation has not been very clear

Replace the following code

a=stack[top--];

if(!a->right)

{

printf("%d ",a->data);

a=NULL;

}

else

a=a->right;

with the following code

a=stack[top--];

if(!a->right)

{

printf("%d ",a->data);

// Print out all the non-leaf nodes whose right child has been processed

while (top >= 0 && stack[top]->right == a){

a = stack[top--];

printf("%d ",a->data);

}

a=NULL;

}

else {

// Not a leaf node, so push it back

stack[++top] = a;

a=a->right;

}

Hope this helps

Wilfred

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