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

char stack[MAX][MAX];
int top = -1;

void push(char ch[]) {
    strcpy(stack[++top],ch);
    
}
char *pop() {
    return stack[top--];
}
int isoperator(char ch) {
    return (isalnum(ch) || ch=='+' || ch=='-' || ch=='*' || ch=='/');
}

int isvalid(char ch) {
    strcpy(stack[++top],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;
        }
        if(isalnum(ch)) {
            char temp[2];
            temp[0] = ch;
            temp[1] = '\n';
            push(temp);
        }
        else if(isoperator(ch)) {
            
            
        }
    }
}