C code to implement Postfix Expression Tree

/* C program to implement postfix expression tree and calculating its value */

#include<stdio.h>
#include<conio.h>
#include<alloc.h>
#include<stdlib.h>
#include<string.h>

struct tree
{
    char data;
    struct tree *left,*right;
};

int top=-1;
struct tree *stack[20];
struct tree *node;

void push(struct tree* node)
{
    stack[++top]=node;
}

struct tree * pop()
{
    return(stack[top--]);
}

/* Calculating the value of postfix expression using recursion */

int cal(struct tree *node)
{
    int ch;
    ch=check(node->data);
    if(ch==1)
    return node->data-48;
    else if(ch==2)
    {
        if(node->data==+)
            return cal(node->left)+cal(node->right);
        if(node->data==-)
            return cal(node->left)-cal(node->right);
        if(node->data==*)
            return cal(node->left)*cal(node->right);
        if(node->data==/)
            return cal(node->left)/cal(node->right);
    }
}

/* Displaying all node in order */

void inorder(struct tree *node)
{
    if(node!=NULL)
    {
    inorder(node->left);
    printf("%c",node->data);
    inorder(node->right);
    }
}

/* Checking operator and operands */

int check(char ch)
{
if(ch==+||ch==-||ch==/||ch==*)
return 2;
else
return 1;
}

/* making simple operand node and pushing into stack */

void operand(char b)
{
    node=(struct tree*)malloc(sizeof(struct tree));
    node->data=b;
    node->left=NULL;
    node->right=NULL;
    push(node);
}

/* making operator node than pop-up two nodes from stack and adding into operator node and finally push node into stack  */

void operators(char a)
{
    node=(struct tree*)malloc(sizeof(struct tree));
    node->data=a;
    node->right=pop();
    node->left=pop();
    push(node);
}

void main()
{
    int i,p,k,ans;
    char s[20];
    clrscr();
    printf("Enter the expression in postfix form
");
    fflush(stdin);
    gets(s);
    k=strlen(s);
    i=0;
    for(i=0;s[i]!=

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