#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#include<string.h>
#define MAX 25

char stack[MAX][MAX];
int top;

void push(char ch[]){
    strcpy(stack[++top],ch);
}
char* pop(){
    return stack[top--];
}
int isOperators( char ch){
    return (ch=='/'||ch=='+'||ch=='-'||ch=='*'||ch=='('||ch==')');
    
}
int isValid(char ch){
    return (isalnum(ch)||ch=='/'||ch=='+'||ch=='-'||ch=='*'||ch=='('||ch=')');
    
}
void fun(char *str){
    int len=strlen(str),ind;
    for(ind=len-1;ind>=0;ind--){
        char ch=str[ind];
        if(!isValid(ch)){
            printf("Invalid input");
            return 0;
        }
        if(isalnum(ch)){
            char temp[2];
            temp[0]=ch;
            temp[1]='\0';
            push(temp);
        }
        else if(isOperator(ch)){
            char ope1[MAX],ope2[MAX],res[MA]
        }
    }
}