15. Valid Number.cpp

/*
Valid Number
============

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

A decimal number or an integer.
(Optional) An 'e' or 'E', followed by an integer.
A decimal number can be split up into these components (in order):

(Optional) A sign character (either '+' or '-').
One of the following formats:
At least one digit, followed by a dot '.'.
At least one digit, followed by a dot '.', followed by at least one digit.
A dot '.', followed by at least one digit.
An integer can be split up into these components (in order):

(Optional) A sign character (either '+' or '-').
At least one digit.
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.

Example 1:
Input: s = "0"
Output: true

Example 2:
Input: s = "e"
Output: false

Example 3:
Input: s = "."
Output: false

Example 4:
Input: s = ".1"
Output: true

Constraints:
1 <= s.length <= 20
s consists of only English letters (both uppercase and lowercase), digits (0-9), plus '+', minus '-', or dot '.'.
*/

class Solution
{
public:
  bool isNumber(string s)
  {
    int pointSeen = 0, numSeen = 0, eSeen = 0, numAfterE = 0;
    for (int i = 0; i < s.size(); ++i)
    {

      if (s[i] >= '0' && s[i] <= '9')
      {
        numSeen = 1;
        numAfterE = 1;
      }
      else if (s[i] == '.')
      {
        if (eSeen || pointSeen)
          return false;
        pointSeen = 1;
      }
      else if (s[i] == 'e' || s[i] == 'E')
      {
        if (eSeen || !numSeen)
          return false;
        eSeen = 1;
        numAfterE = 0;
      }
      else if (s[i] == '+' || s[i] == '-')
      {
        if (i != 0 && s[i - 1] != 'e')
          return false;
      }
      else
        return false;
    }
    return numSeen && numAfterE;
  }
};