# 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


*/
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jaywin.gitbook.io/leetcode/solutions/0065-valid-number.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.