infix to postfix(normal)

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

struct node{
    int data;
    struct node* next;
};
struct node* top= (struct node*)malloc(sizeof(node));
string a,b;
char aa;
int i,j,k,l,x;

void push(int n)
{
    struct node* newnode=(struct node*)malloc(sizeof(node));
    newnode->data=n;
    newnode->next=top;
    top=newnode;
}
char peek()
{
   return top->data;
}
bool isEmpty()
{
    if(top->next==NULL)
        return 1;
    else
        return 0;
}
void pop()
{
    if(isEmpty()==0)
        top=top->next;
}
/*void traversal(struct node* dummy)
{
    while(dummy->next!=NULL)
    {
        cout <<dummy->data <<" ";
        dummy=dummy->next;
    }
    cout <<endl;
}*/
int precidence(char c)
{
    if(c=='*')
       return 2;
    else if(c=='/')
        return 2;
    else if(c=='+')
        return 1;
    else if(c=='-')
        return 1;
}
int main()
{
    top->next=NULL;

    cout<<"Infix Expression: ";
    cin >>a;
    for(i=0; i<a.size(); i++)
    {
        if(a[i]>='0' && a[i]<='9')
            b += a[i];
        else
        {
           while(precidence(a[i])<precidence(peek()))
           {
                b += peek();
                pop();
           }
           push(a[i]);
        }
    }
    while(isEmpty() == 0)
    {
        b += peek();
        pop();
    }
   cout <<"Postfix Expression: "<< b <<endl;
   for(i=0; i<b.size(); i++)
    {
        if(b[i] >= '0' && b[i] <= '9')
            push(b[i]);
        else
        {
            int p = peek()-'0';
            pop();
            int q = peek()-'0';
            pop();

            if(b[i]=='*')
                x = q*p;
            else if(b[i]=='/')
                x = q/p;
            else if(b[i]=='+')
               x = q+p;
            else if(b[i]=='-')
                x = q-p;
             aa = x+'0';
             push(aa);
        }
    }
    cout<<"Result: "<< x <<endl;
    return 0;
}