LeetCode 520 Detect Capital

Input: "USA"
Output: True

Input: "FlaG"
Output: False

class Solution {
    public boolean detectCapitalUse(String word) {
        if (word.length() < 2) {
            return true;
        }
        char[] chars = word.toCharArray();
        int lower = 0, upper = 0;
        for (int i = 0; i < chars.length; i++) {
            if (isLowerCaseLetter(chars[i])) {
                lower++;
            } else {
                upper++;
            }
        }
        
        return lower == chars.length || upper == chars.length ||
                (isUpperCaseLetter(chars[0]) && lower == chars.length - 1);
    }
    
    public boolean isUpperCaseLetter(char c) {
        return c >= 65 && c <= 90;
    }
    
    public boolean isLowerCaseLetter(char c) {
        return c >= 97 && c <= 122;
    }
}