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.

Solution：check

思路：
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

class Solution {
    public boolean isNumber(String s) {
        
        // eliminates leading and trailing spaces
        s = s.trim();

        boolean pointSeen = false;
        boolean eSeen = false;
        boolean numberSeen = false;
        boolean numberAfterE = true;
        for(int i=0; i<s.length(); i++) {
            if(Character.isDigit(s.charAt(i))) {
                numberSeen = true;
                numberAfterE = true;
            } else if(s.charAt(i) == '.') {
                if(eSeen || pointSeen) {
                    return false;
                }
                pointSeen = true;
            } else if(s.charAt(i) == 'e') {
                if(eSeen || !numberSeen) {
                    return false;
                }
                numberAfterE = false;
                eSeen = true;
            } else if(s.charAt(i) == '-' || s.charAt(i) == '+') {
                if(i != 0 && s.charAt(i-1) != 'e') {
                    return false;
                }
            } else {
                return false;
            }
        }

        return numberSeen && numberAfterE;
    }
}

test case

    test(1, "123", true);
    test(2, " 123 ", true);
    test(3, "0", true);
    test(4, "0123", true);  //Cannot agree
    test(5, "00", true);  //Cannot agree
    test(6, "-10", true);
    test(7, "-0", true);
    test(8, "123.5", true);
    test(9, "123.000000", true);
    test(10, "-500.777", true);
    test(11, "0.0000001", true);
    test(12, "0.00000", true);
    test(13, "0.", true);  //Cannot be more disagree!!!
    test(14, "00.5", true);  //Strongly cannot agree
    test(15, "123e1", true);
    test(16, "1.23e10", true);
    test(17, "0.5e-10", true);
    test(18, "1.0e4.5", false);
    test(19, "0.5e04", true);
    test(20, "12 3", false);
    test(21, "1a3", false);
    test(22, "", false);
    test(23, "     ", false);
    test(24, null, false);
    test(25, ".1", true); //Ok, if you say so
    test(26, ".", false);
    test(27, "2e0", true);  //Really?!
    test(28, "+.8", true);  
    test(29, " 005047e+6", true);  //Damn = =|||