Identifying palindromes using stacks

Question

I wrote the following code to check whether a string is a palindrome using stacks. I'm not getting the correct output whenever there are two same continuous characters in the string. For example, the code does say exe is a palindrome. But it says ee is not a palindrome.

int is_palindrome(char str[]) {
  int j, top = -1;
  char stk_item, s[30];

  for (j = 0; j < strlen(str); j++)
    s[top++] = str[j];
  for (j = 0; j < strlen(str); j++) {
    stk_item = s[--top];
    if (str[j] != stk_item) return 0; // is not a palindrome
  }

  return 1; // is a palindrome
}

What may be the issue?

Answer 1

You confused the pre-increment and post-increment operators.

In C, x++ means "increment the value of x after this statement" and ++x means "first increment the value of x and then execute the statement."

The following code should work:

int is_palindrome(char str[]) {
  int j, top = -1;
  char stk_item, s[30];

  for (j = 0; j < strlen(str); j++)
    s[++top] = str[j];
  for (j = 0; j < strlen(str); j++) {
    stk_item = s[top--];
    if (str[j] != stk_item) return 0;
  }

  return 1;
}