A valid number can be split up into these components (in order):

  1. A decimal number or an integer.
  2. (Optional) An 'e' or 'E', followed by an integer.

A decimal number can be split up into these components (in order):

  1. (Optional) A sign character (either '+' or '-').
  2. One of the following formats:
    1. One or more digits, followed by a dot '.'.
    2. One or more digits, followed by a dot '.', followed by one or more digits.
    3. A dot '.', followed by one or more digits.

An integer can be split up into these components (in order):

  1. (Optional) A sign character (either ‘+’ or ‘-‘).
  2. One or more digits.

For example, all the following are valid numbers: ["2", "0089", "-0.1", "+3.14", "4.", "-.9", "2e10", "-90E3", "3e+7", "+6e-1", "53.5e93", "-123.456e789"], while the following are not valid numbers: ["abc", "1a", "1e", "e3", "99e2.5", "--6", "-+3", "95a54e53"].

Given a string s, return true if s is a valid number.

Test Cases

Example 1:

Input: s = "e"
Output: false

Example 2:

Input: s = "5."
Output: true

Solution

class Solution {
public:
    bool isNumber(string s) {
        bool num = false, dec = false, sign = false, exp = false;
        for(int i=0; i<s.size(); i++) {
            char c = s[i];
            if (c >= '0' && c <= '9') {
                num = true;
            } else if (c == 'e' || c == 'E') {
                if (exp || !num) {
                    return false;
                }
                exp = true;
                sign = false;
                dec = false;
                num = false;
            } else if (c == '+' || c == '-') {
                if (sign || num || dec) {
                    return false;
                }
                sign = true;
            } else if (c == '.') {
                if (dec || exp) {
                    return false;
                }
                dec = true;
            } else {
                return false;
            }
        }
        return num;
    }
};
class Solution {
    public boolean isNumber(String s) {
        s = s.trim();
        boolean eseen = false, numseen = false, pointseen = false;
        for(int i=0; i<s.length(); i++) {
            if ('0' <= s.charAt(i) && s.charAt(i) <= '9') {
                numseen = true;
            } else if (s.charAt(i) == '.') {
                if (pointseen || eseen) {
                    return false;
                }
                pointseen = true;
            } else if (s.charAt(i) == 'e' || s.charAt(i) == 'E') {
                if (eseen || !numseen) {
                    return false;
                }
                numseen = 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 numseen;
    }
}
class Solution:
    def isNumber(self, s: str) -> bool:
        num, exp, dec, sign = [False]*4
        for c in s:
            if '0' <= c <= '9':
                num = True
            elif c == 'e' or c == 'E':
                if not num or exp:
                    return False
                exp = True
                sign = False
                dec = False
                num = False
            elif c == '.':
                if exp or dec:
                    return False
                dec = True
            elif c == '+' or c == '-':
                if sign or num or dec:
                    return False
                sign = True
            else:
                return False
        return num
Time Complexity: O(n)
Space Complexity: O(1)