My code:

public class Solution {
    public boolean isNumber(String s) {
        int i = 0;
        int e = s.length() - 1;
        while (i <= e && s.charAt(i) == ' ') {
            i++;
        }
        if (i > e) {
            return false;
        }
        while (e >= i && s.charAt(e) == ' ') {
            e--;
        }
        
        if (s.charAt(i) == '+' || s.charAt(i) == '-') {
            i++;
        }
        
        boolean num = false;
        boolean dot = false;
        boolean exp = false;
        
        while (i <= e) {
            char curr = s.charAt(i);
            if (Character.isDigit(curr)) {
                num = true;
            }
            else if (curr == '.') {
                if (exp || dot) {
                    return false;
                }
                dot = true;
            }
            else if (curr == 'e') {
                if (exp || !num) {
                    return false;
                }
                exp = true;
                num = false;
            }
            else if (curr == '+' || curr == '-') {
                if (s.charAt(i - 1) != 'e') {
                    return false;
                }
            }
            else {
                return false;
            }
            i++;
        }
        
        return num;
    }
}

reference:
http://www.jiuzhang.com/solutions/valid-number/

这种题，其实一点意义都没。连 valid number 的定义都没。
死记下最简单的答案吧。

Anyway, Good luck, Richardo! — 10/15/2016