65. Valid Number

Validate if a given string can be interpreted as a decimal number.

Some examples: "0" => true " 0.1 " => true "abc" => false "1 a" => false "2e10" => true " -90e3 " => true " 1e" => false "e3" => false " 6e-1" => true " 99e2.5 " => false "53.5e93" => true " --6 " => false "-+3" => false "95a54e53" => false

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one. However, here is a list of characters that can be in a valid decimal number:

• Numbers 0-9

• Exponent - "e"

• Positive/negative sign - "+"/"-"

• Decimal point - "."

Of course, the context of these characters also matters in the input.

bool isNumber(string s) { // time: O(n); space: O(1)
    bool num = false, numAfterE = true, sign = false, dot = false, exp = false;
    int n = s.length();
    for (int i = 0; i < n; ++i) {
        if (s[i] == ' ') {
            if (i + 1 < n && s[i + 1] != ' ' && (num || sign || dot || exp)) return false;
        } else if (s[i] == '+' || s[i] == '-') {
            if (i > 0 && s[i - 1] != ' ' && s[i - 1] != 'e' && s[i - 1] != 'E') return false;
            sign = true;
        } else if (s[i] >= '0' && s[i] <= '9') {
            num = true;
            numAfterE = true;
        } else if (s[i] == '.') {
            if (dot || exp) return false;
            dot = true;
        } else if (s[i] == 'e' || s[i] == 'E') {
            if (!num || exp) return false;
            exp = true;
            numAfterE = false;
        } else return false;
    }
    return num && numAfterE;
}