# 0065. Valid Number

https://leetcode.com/problems/valid-number

## Description

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**_.

**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 `'.'`.

## ac

The key is to understand the rules

```java
class Solution {
    public boolean isNumber(String s) {
        // edge cases
        if (s == null) return false;
        s = s.trim();
        if (s.length() == 0) return false;

        boolean signSeen = false;
        boolean dotSeen = false;
        boolean eSeen = false;
        boolean numberSeen = false;
        int n  =  s.length();

        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);

            if (!isValid(c)) return false;  // not a valid char

            // sign: first, right after e/E, not last，twice
            if (c == '+' || c == '-') {
                if (i > 0 && (s.charAt(i-1) != 'e' && s.charAt(i-1) != 'E') 
                    || i == n - 1)
                    return false;
                signSeen = true;
            }

            // dot: once, not after e, last with numberSeen
            if (c == '.') {
                if (dotSeen || eSeen || i == n - 1 && !numberSeen) return false;
                dotSeen = true;
            }

            // e/E: after number, once, not last
            if (c == 'e' || c == 'E') {
                if (!numberSeen || eSeen || i == n - 1) return false;
                eSeen = true;
            }

            // numbers
            if (c >= '0' && c <= '9') {
                numberSeen = true;
            }
        }

        return true;
    }

    private boolean isValid(char c) {
        if (c == '.' || c == '+' || c == '-' || c == 'e' || c == 'E' || c >= '0' && c <= '9')
            return true;

        return false;
    }
}

/*
trim()
special char: .  e  +-  number
signSeen, first, after e, not last
dotSeen, once, not last, not after e
eSeen, after number, once, not last
numberSeen


*/
```
