valid-number.java

class Solution {
    public boolean isNumber(String s) {
        s = s.trim();
        if (s.isEmpty()) return false;

        boolean num = false, dot = false, exp = false;

        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);

            if (Character.isDigit(c)) num = true;
            else if (c == '+' || c == '-') {
                if (i > 0 && s.charAt(i - 1) != 'e' && s.charAt(i - 1) != 'E') return false;
            }
            else if (c == '.') {
                if (dot || exp) return false;
                dot = true;
            }
            else if (c == 'e' || c == 'E') {
                if (exp || !num) return false;
                exp = true;
                num = false; // reset for exponent part
            }
            else return false;
        }

        return num;
    }
}