expr eval using postfix/rev polish

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <stdbool.h>
#define NMAX 101
#define SIGMA ('z' - 'a' + 1)

int opPrec(char op) {
  if (strchr("+-", op)) {
    return 1;
  }
  if (strchr("*/", op)) {
    return 2;
  }
  return 0;//not op
}

char *toPostfix(const char infix[]) {
  const int n = strlen(infix);
  int top = -1;
  static char res[NMAX], st[NMAX];
  for (int i = 0; i < n; i++) {
    if (infix[i] == '(') {
      st[++top] = infix[i];
    } else if (infix[i] == ')') {
      while (top >= 0 && st[top] != '(') {
        strncat(res, &st[top--], 1);
      }
      top--;
    } else if (opPrec(infix[i])) {
      while (top >= 0 && opPrec(infix[i]) <= opPrec(st[top])) {
        strncat(res, &st[top--], 1);
      }
      st[++top] = infix[i];
    } else if (isalpha(infix[i])) {
      strncat(res, &infix[i], 1);
    }
  }

  while (top >= 0) {
    strncat(res, &st[top--], 1);
  }
  return res;
}

int computeOp(int a, int b, char op) {
  switch(op) {
  case '+':
    return a + b;
  case '-':
    return a - b;
  case '*':
    return a * b;
  case '/':
    return a / b;
  }
  return 0;//to supress warning
}

int evalPostfixEx(const char postfix[], const int valOf[SIGMA]) {
  static int st[NMAX];
  int top = -1;
  for (int i = 0; postfix[i]; i++) {
    if (isalpha(postfix[i])) {
      st[++top] = valOf[postfix[i] - 'a'];
    } else {
      int op2 = st[top--];
      int op1 = st[top--];
      int res = computeOp(op1, op2, postfix[i]);
      st[++top] = res;
    }
  }
  return st[top];
}

int cntLetters(const char s[]) {
  static bool used[SIGMA];
  for (int i = 0; s[i]; i++) {
    used[s[i] - 'a'] = true;
  }
  int cnt = 0;
  for (int let = 0; let < SIGMA; let++) {
    cnt += used[let];
  }
  return cnt;
}

int main(void) {
  static char infix[NMAX];
  static int valOf[SIGMA];
  if (!fgets(infix, NMAX, stdin)) {
    perror("");
    exit(-1);
  }

  int nrLetters = cntLetters(infix);
  for (int i = 0; i < nrLetters; i++) {
    scanf("%d", &valOf[i]);
  }


  char *postfix = toPostfix(infix);
  puts(postfix);
  printf("%d\n", evalPostfixEx(postfix, valOf));//*/
  return 0;
}