Expression Tree

#include <stdio.h>
#include <stdlib.h>
struct stack{
    char data;
    struct stack *left, *right, *bottom;
} *top = NULL;
void push(struct stack *newNode){
    if(top) newNode->bottom = top;
    top = newNode;
}
struct stack* pop(){
    struct stack *target = top;
    top = top->bottom;
    target->bottom = NULL;
    return target;
}
void display(struct stack *help){
    if(help){
        display(help->left);
        display(help->right);
        printf("%c", help->data);
    }
}
int main(){
    char postfix[20];
    printf("Enter the expression(oprands can be A-Z or 0-9): ");
    gets(postfix);
    for (int i = 0; postfix[i] != '\0'; i++){
        struct stack *newNode = (struct stack*) malloc(sizeof(struct stack));
        newNode->data = postfix[i]; newNode->bottom = NULL;
        if((newNode->data >= 'A' && newNode->data <= 'Z')||(newNode->data >= '0' && newNode->data <= '9')){
            newNode->left = newNode->right = NULL;
            push(newNode);
        }
        else{
            if(!top->bottom) goto end;
            newNode->right = pop(); newNode->left = pop();
            push(newNode);
        }
    }
    if(!top->bottom) display(top);
    else end: printf("\nNot a valid postfix expression.");
    return 0;
}