Reverse Polish Notation (RPN)

#include<stdio.h>
#include <stdlib.h>
#include<string.h>
char stack[100];
short top = -1;
char cima()
{
    return top == -1 ? 0 : stack[top];
}

char pop()
{
    char s = top == -1 ? 0 : stack[top];
    stack[top--] = 0;
    return s;
}

void push(char x)
{
    if (top != 99)
        stack[++top] = x;
}


int par=0;
int priority(char x)
{
    switch (x)
    {
    case '(':
        par++;return 0;
    case ')':
        if(--par<0){
            printf("\nUnbalanced parenthesis\n");
            exit(1);
        }return 1;
    case '=':
        return 2;
    case '+':
    case '-':
        return 3;
    case '*':
    case '/':
        return 4;
    case '^':
        return 5;
    }
    char z[]="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    for(int a=0;a<strlen(z);a++)if(z[a]==x)return-1;
    return -2;
}

void printOp(char z[], int pun)
{
    if (pun)
    {
        z[pun] = 0;
        printf("%s ", z);
    }
}
void spellCheck(char x[]){
    int p,last=0;
    for (int a = 0; a < strlen(x); a++){
        p = priority(x[a]);
        if(p>2&&last>2){
            printf("\nConsecutive operator\n");
            exit(2);
        }
        last=p;
    }
    if(par!=0){
        printf("\nUnbalanced parenthesis\n");
        exit(1);
    }
}

void parse(char x[])
{
    char z[50];
    short pun = 0;
    int p;
    printf("Notazione polacca inversa: ");
    for (int a = 0; a < strlen(x); a++)
    {
        p = priority(x[a]);
        if (p==-2);
        else if (p == -1)
            z[pun++] = x[a];
        else
        {
            printOp(z, pun);
            pun = 0;
            if (p == 0)
                push(x[a]);
            else if (p == 1)
            {
                while (cima() != '(')
                {
                    printf("%c ", pop());
                }
                pop();

            }
            else if (priority(cima()) < p)
                push(x[a]);
            else if (priority(cima()) >= p)
            {
                while (priority(cima()) >= p && cima() != '(' && cima() != 0)
                    printf("%c ", pop());
                push(x[a]);
            }
        }

    }
    printOp(z, pun);
    while (cima() != 0)
        printf("%c ", pop());
    printf("\n");
}

int main()
{
    char str[200];
    printf("Inserire espressione: ");
    gets(str);
    spellCheck(str);
    parse(str);
}