二叉树的中序遍历遍历后给出垃圾值和分段错误

Question

I am trying to traverse a binary tree using a stack, the traversal is successful but the program at the end shows garbage values and segmentation fault error. I think it's not going to the top again and again but I m not able to fix this.

struct Node
{
    int data;
    Node *left;
    Node *right;
    Node(int x) 
    {
        data = x;
        left = NULL;
        right = NULL;
    }
};

Node *STACK[10] = { NULL };
int TOP = -1;

void push(Node *ptr)
{
    if(TOP < 10) {
        STACK[++TOP] = ptr;
    }
}

void stackTraversal(Node *root)
{
    Node *ptr = root; bool flag = false;
    top:
    while(ptr != NULL)
    {
        push(ptr);
        ptr = ptr->left;
    }

    ptr = STACK[TOP];
    TOP--;
    while(ptr != NULL)
    {
        cout << ptr->data << " ";
        if(ptr->right != NULL)
        {
            ptr = ptr->right;
            flag = true;
            break;
        }
        ptr = STACK[TOP];
        TOP--;
    }
    if(flag)
        goto top;
    cout << "\nTHE END\n"; 
}

int main(int argc, char const *argv[])
{
    Node *R = new Node(2); 
    Node *a = new Node(0);
    Node *b = new Node(1);
    Node *c = new Node(4);
    Node *d = new Node(5);
    Node *e = new Node(3);

    R->right = c;
    R->left = a;

    a->right = b;

    c->right = d;
    c->left = e;

    stackTraversal(R);
    cout << endl;
    return 0;
}

it is giving the following ouput.

Output:- 0 1 2 3 4 5 -786491 segmentation fault (core dumped)

Answer 1

As you can see from your output, you have traversed all elements, and the last element visited is d .

Now you are in this block, where ptr points to d :

        if(ptr->right != NULL)
        {
            ptr = ptr->right;
            flag = true;
            break;
        }
        ptr = STACK[TOP];

Ok, d has no children, you don't enter the if block, and the next node you plan to visit is... STACK[-1] .

Revise you algorithm. I recommend to avoid using goto as long as this is a notoriously bad practice.

Answer 2

Your algorithm pops from the stack assuming there would always be something. But how should ptr then become NULL to end the second loop?

As a consequence, your stack counter goes into the negative causing UB which (fortunately) expresses itself as a runtime error.

Improvement 1: detect the end

You made the effort to write a push() . Make the extra effort of writing a pop() . Make sure that either your algorithm checks for an empty stack or that pop() returns nullptr if the stack is empty:

Node* pop()
{
    if (TOP>=0) { 
        return STACK[TOP--];
    }
    else {
        return nullptr;
    }
} 

Of course, replace the folowing sequences with ptr = pop(); :

ptr = STACK[TOP];
TOP--;

All the pop() are followed by the pointer being checked for null, except for use of ptr after the goto . So you need to add an extra condition to avoid this situation:

if(flag && ptr)
    goto top;

When you then run the programme, it will display the items, but you'll see that an underflow ends the process.

Improvement 2: Get rid of the goto

We are in the 21st century: goto is no longer a fatality, and should better be avoided in C++ if possible .

An outer loop could elegantly replace the goto. You can then get rid of the flag as well:

void stackTraversal(Node *root)
{
    ...
    while(ptr)   // outer loop: no more goto 
    {
        while(ptr) {        // push the full left
            push(ptr);
            ptr = ptr->left;
        }
        ptr = pop();       // pop top node 
        while(ptr)        
        {
            cout << ptr->data << " ";
            cout.flush();   // in case of weird things happening afterwards
            if(ptr->right) {    // explore its right branch
                ptr = ptr->right;
                break;          // this goes back to outer loop
            }
            else ptr = pop();
        }
    }
    cout << "\nTHE END\n"; 
}

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