https://leetcode.com/problems/valid-number/

valid-number.java

class Solution {
    public boolean isNumber(String s) {
        int idx = 0;
        boolean seenE = false;
        boolean seenDot = false;
        boolean seenSign = false;
        boolean seenNum = false;
        s = s.trim();
        
        while(idx < s.length()) {
            if (s.charAt(idx) == 'e') {
                if (seenE || !seenNum) return false;
                // end of a num
                if (seenDot && !seenNum) {
                    return false;
                }
                if (seenSign && !seenNum) {
                    return false;
                }
                seenE = true;
                // another num after e
                seenNum = false;
                seenSign = false;
                seenDot = false;
            } else if (s.charAt(idx) == '.') {
                if (seenDot || seenE) return false;
                seenDot = true;
            } else if (s.charAt(idx) == '+' || s.charAt(idx) == '-') {
                if (seenNum || seenDot || seenSign) {
                    return false;
                }
                seenSign = true;
            } else if (Character.isDigit(s.charAt(idx))) {
                seenNum = true;
            } else {
                return false;
            }
            idx++;
        }
        if (!seenNum) {
            return false;
        }
        if (seenDot && !seenNum) {
            return false;
        }
        if (seenSign && !seenNum) {
            return false;
        }
        return true;
    }
}