Prefix Expression Evaluation

#include <bits/stdc++.h>
using namespace std;

#define mx 30
int i(0);

class node
{
public:
    int info;
    node *link;
};

node *head = NULL, *tmp = NULL;

void push(int val)
{
    node *ptr = new node();
    ptr->info = val;
    ptr->link = head;
    head = ptr;
    i++;
}

int top()
{
    return head->info;
}

int pop()
{
    int val = head->info;
    head = head->link;
    return val;
}

bool isEmpty()
{
    if(head == NULL)
    {
        cout<<"\nStack Underflow!!!!\n\n";
        return true;
    }
    return false;
}

bool isFull()
{
    if(i >= mx)
    {
        cout<<"\nStack Overflow!!!\n\n";
        return true;
    }
    return false;
}

bool isOperator(char ch)
{
    if(ch == '+' || ch == '-' || ch == '*' || ch == '/')
        return true;
    return false;
}

bool isNumeric(char ch)
{
    if(ch >= '0' && ch <= '9')
        return true;
    return false;
}

int perform(int a, int b, char ch)
{
    if(ch == '+')
        return a+b;
    if(ch == '-')
        return a-b;
    if(ch == '*')
        return a*b;
    if(ch == '/')
        return a/b;
}

int evaluatePrefix(string exp)
{
    int i, j, k, sz, op1, op2;
    sz = exp.size() - 1;
    for(i = sz; i >= 0; i--)
    {
        if(exp[i] == ' ' || exp[i] == ',')
            continue;
        else if(isOperator(exp[i]))
        {
            op1 = pop();
            op2 = pop();
            k = perform(op1, op2, exp[i]);
            push(k);
        }
        else if(isNumeric(exp[i]))
        {
            int a, b, c, d, oprnd;
            oprnd = 0;
            d = 1;
            for(; i >= 0 && isNumeric(exp[i]); i--, d *= 10)
                oprnd = (exp[i] - '0') * d + oprnd;
            i++;
            push(oprnd);
        }
        else
        {
            cout<<"The expression contains wrong symbol.\n";
            exit(0);
        }
    }
    return k;
}

int main()
{
    int i, j, k, n;
    string str;
    getline(cin, str);
    cout<<evaluatePrefix(str)<<endl;
}