C++ Program to Implement Expression Tree Algorithm

Code:

#include    iostream
#include    conio.h
using namespace std;
struct tree
{
    char data;
    tree *l, *r;
}*root = NULL, *p = NULL, *t = NULL, *y = NULL;
struct node
{
       tree *pt;
       node *next;
}*top = NULL, *q = NULL, *np = NULL;
void push(tree *ptr)
{
    np = new node;
    np->pt = ptr;
    np->next = NULL;
    if (top == NULL)
    {
        top = np;
    }
    else
    {
        q = top;
        top = np;
        np->next = q;
    }
}
tree *pop()
{
    if (top == NULL)
    {
        cout<<"underflow\n";
    }
    else
    {
        q = top;
        top = top->next;
        return(q->pt);
        delete(q);
    }
}
void oprnd_str(char val)
{
    if (val >= 48 && val <= 57)
    {
        t = new tree;
        t->data = val;
        t->l = NULL;
        t->r = NULL;
        push(t);
    }
    else if (val >= 42 && val <= 47)
    {
        p = new tree;
        p->data = val;
        p->l = pop();
        p->r = pop();
        push(p);
    }
}
char pstorder(tree *w)
{
    if (w != NULL)
    {
        pstorder(w->l);
        pstorder(w->r);
        cout<data;
    }
}
int main()
{
    char a[15];
    int i;
    int j = -1;
    cout<<"enter the value of character string\n";
    cin>>a;
    i = strlen(a);
    while (i >= 0)
    {
        i--;
        oprnd_str(a[i]);
    }
    cout<<"displaying in postorder\n";
    pstorder(pop());
    getch();
}


Output:

enter the value of character string
-+-5/763*48
displaying in postorder
576/-3+48*-


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