Valid number

Validate if a given string is numeric.

Some examples:
"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

思路：A e B

A 可以是小数，整数 e 后面必须紧跟 +-， 后面B必须是整数，最后返回numberseen；因为扫描最后就是number；

class Solution {
    public boolean isNumber(String s) {
        s = s.trim().toLowerCase();
        // AeB
        boolean eseen = false;
        boolean numberseen = false;
        boolean dotseen = false;
        for(int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if('0' <= c && c <= '9') {
                numberseen = true;
            } else if(c == 'e') {
                if(eseen || !numberseen) {
                    return false;
                }
                eseen = true;
                numberseen = false; // 注意这里看见了e之后，numberseen = false;
            } else if(c == '.') {
                if(eseen || dotseen) {
                    return false;
                }
                dotseen = true;
            } else if(c == '+' || c == '-') {
                if(i >= 1 && s.charAt(i - 1) != 'e') {
                    return false;
                }
            } else {
                return false;
            }
        }
        return numberseen; //最后返回是否看见number；
    }
}