Leetcode QOTD:- 3120. Count the Number of Special Characters I

1. Use two integers as bit masks:
   • lower stores lowercase letters present.
   • upper stores uppercase letters present.
2. Traverse each character in the string:
   • If lowercase, set its corresponding bit in lower.
   • If uppercase, set its corresponding bit in upper.
3. Perform lower & upper:
   • Common set bits represent characters present in both lowercase and uppercase forms.
4. Use Integer.bitCount() to count total common bits and return the answer.

class Solution {
    public int numberOfSpecialChars(String word) {
        int lower = 0, upper = 0;

        for (char c : word.toCharArray()) {
            if (Character.isLowerCase(c))
                lower |= 1 << (c - 'a');
            else
                upper |= 1 << (c - 'A');
        }

        return Integer.bitCount(lower & upper);
    }
}