C++ Program to Implement Expression Tree Algorithm

This C++ program, implements the expression tree algorithm which consists of operands as terminal nodes and operators as root or sub root nodes.

Here is the source code of the C++ program which takes the prefix expression as an input and generates the corresponding expression tree traversed in postorder. This C++ program is successfully compiled and run on DevCpp, a C++ compiler. The program output is also shown below.

```
/*
```

* C++ Program to Implement Expression Tree Algorithm

```
*/
```
```
#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
```
```
{
```
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       tree *pt;
```
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       node *next;
```
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}*top = NULL, *q = NULL, *np = NULL;
```
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void push(tree *ptr)
```
```
{
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    np = new node;
```
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    np->pt = ptr;
```
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    np->next = NULL;
```
```
    if (top == NULL)
```
```
    {
```
```
        top = np;
```
```
    }
```
```
    else
```
```
    {
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        q = top;
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        top = np;
```
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        np->next = q;
```
```
    }
```
```
}
```
```
tree *pop()
```
```
{
```
```
    if (top == NULL)
```
```
    {
```
```
        cout<<"underflow\n";
```
```
    }
```
```
    else
```
```
    {
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        q = top;
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        top = top->next;
```
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        return(q->pt);
```
```
        delete(q);
```
```
    }
```
```
}
```
```
void oprnd_str(char val)
```
```
{
```
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    if (val >= 48 && val <= 57)
```
```
    {
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```
        t = new tree;
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        t->data = val;
```
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        t->l = NULL;
```
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        t->r = NULL;
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        push(t);
```
```
    }
```
```
    else if (val >= 42 && val <= 47)
```
```
    {
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        p = new tree;
```
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        p->data = val;
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        p->l = pop();
```
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        p->r = pop();
```
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        push(p);
```
```
    }
```
```
}
```
```
char pstorder(tree *w)
```
```
{
```
```
    if (w != NULL)
```
```
    {
```
```
        pstorder(w->l);
```
```
        pstorder(w->r);
```
```
        cout<<w->data;
```
```
    }
```
```
}
```
```
int main()
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```
{
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```
    char a[15];
```
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    int i;
```
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    int j = -1;
```

    cout<<"enter the value of character string\n";

```
    cin>>a;
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    i = strlen(a);
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```
    while (i >= 0)
```
```
    {
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```
        i--;
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        oprnd_str(a[i]);
```
```
    }
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    cout<<"displaying in postorder\n";
```
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    pstorder(pop());
```
```
    getch();
```
```
}
```

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

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