Evo of postfix expression

//evolution of postfix expression
#include<stdio.h>
#include<conio.h>
#include<MATH.h>
#include<string.h>
char postfix[50],tp[50];
int stack[50],top= (-1),ans,a,b;
void alert()
{
    printf("\nþûþ notice :==\n \"If you Enter NUMBER then Enter the space after the number is complete\" \n LIKe : \"3 33 +\"  ");
}
void push(int ch)
{
    top++;
    stack[top]=ch;
}
int pop()
{
    int ch;
    ch=stack[top];
    top--;
    return ch;
}
int main()
{
    int i,j,ans,a,b,amt,l;
    char c[20];
    clrscr();
    alert();
    printf("\n ENter the postfix Expression: ");
    gets(postfix);

    for(i=0;i<strlen(postfix);i++)
    {
        j=0;
        while(postfix[i]>=48 && postfix[i]<=57)
        {
            c[j]=postfix[i];
            i++;
            j++;

        }
        c[j]=NULL;

        amt=atoi(c);

        if(amt!=0)
        {

            push(amt);
        }

        switch(postfix[i])
        {
            case ' ':
                printf("\n");
                break;

            case '^':
                a=stack[top];
                top--;
                b=stack[top];
                ans=pow(b,a);
                stack[top]=ans;
                break;

            case '*':
                a=stack[top];
                top--;
                b=stack[top];
                ans=b*a;
                stack[top]=ans;
                break;

            case '/':
                a=stack[top];
                top--;
                b=stack[top];
                ans=b/a;
                stack[top]=ans;
                break;

            case '+':
                a=stack[top];
                top--;
                b=stack[top];
                ans=b+a;
                stack[top]=ans;
                break;

            case '-':
                a=stack[top];
                top--;
                b=stack[top];
                ans=b-a;
                stack[top]=ans;
                break;

        }

    }
    printf("\nEVO of POSTFIX expression ==> %d",stack[top]);
    getch();

    return 0;
}