valid-number

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Algorithms

Given a string <code>s</code>, return whether <code>s</code> is a valid number. 
 
For example, all the following are valid numbers: <code>&quot;2&quot;, &quot;0089&quot;, &quot;-0.1&quot;, &quot;+3.14&quot;, &quot;4.&quot;, &quot;-.9&quot;, &quot;2e10&quot;, &quot;-90E3&quot;, &quot;3e+7&quot;, &quot;+6e-1&quot;, &quot;53.5e93&quot;, &quot;-123.456e789&quot;</code>, while the following are not valid numbers: <code>&quot;abc&quot;, &quot;1a&quot;, &quot;1e&quot;, &quot;e3&quot;, &quot;99e2.5&quot;, &quot;--6&quot;, &quot;-+3&quot;, &quot;95a54e53&quot;</code>.

Formally, a&nbsp;valid number is defined using one of the following definitions:

<ol>
	<li>An integer number followed by an optional exponent.</li>
	<li>A decimal number followed by an optional exponent.</li>
</ol>

An integer number is defined with an optional sign <code>&#39;-&#39;</code> or <code>&#39;+&#39;</code> followed by digits.

A decimal number is defined with an optional sign <code>&#39;-&#39;</code> or <code>&#39;+&#39;</code> followed by one of the following definitions:

<ol>
	<li>Digits followed by a dot <code>&#39;.&#39;</code>.</li>
	<li>Digits followed by a dot <code>&#39;.&#39;</code> followed by digits.</li>
	<li>A dot <code>&#39;.&#39;</code> followed by digits.</li>
</ol>

An exponent is defined with an exponent notation <code>&#39;e&#39;</code> or <code>&#39;E&#39;</code> followed by an integer number.

The digits are defined as one or more digits.

&nbsp;
Example 1:

<div class="example-block">
Input: s = &quot;0&quot;

Output: true
</div>

Example 2:

<div class="example-block">
Input: s = &quot;e&quot;

Output: false
</div>

Example 3:

<div class="example-block">
Input: s = &quot;.&quot;

Output: false
</div>

&nbsp;
Constraints:

<ul>
	<li><code>1 &lt;= s.length &lt;= 20</code></li>
	<li><code>s</code> consists of only English letters (both uppercase and lowercase), digits (<code>0-9</code>), plus <code>&#39;+&#39;</code>, minus <code>&#39;-&#39;</code>, or dot <code>&#39;.&#39;</code>.</li>
</ul>

isNumber

Valid Number

String to Integer (atoi)

string-to-integer-atoi

字符串转换整数 (atoi)

String

给定一个字符串&nbsp;<code>s</code>&nbsp;，返回&nbsp;<code>s</code>&nbsp;是否是一个 有效数字。

例如，下面的都是有效数字：<code>"2", "0089", "-0.1", "+3.14", "4.", "-.9", "2e10", "-90E3", "3e+7", "+6e-1", "53.5e93", "-123.456e789"</code>，而接下来的不是：<code>"abc", "1a", "1e", "e3", "99e2.5", "--6", "-+3", "95a54e53"</code>。

一般的，一个 有效数字&nbsp;可以用以下的规则之一定义：

<ol>
	<li>一个 整数 后面跟着一个 可选指数。</li>
	<li>一个 十进制数 后面跟着一个&nbsp;可选指数。</li>
</ol>

一个 整数 定义为一个&nbsp;可选符号&nbsp;<code>'-'</code>&nbsp;或&nbsp;<code>'+'</code>&nbsp;后面跟着 数字。

一个 十进制数&nbsp;定义为一个&nbsp;可选符号&nbsp;<code>'-'</code>&nbsp;或&nbsp;<code>'+'</code>&nbsp;后面跟着下述规则：

<ol>
	<li>数字&nbsp;后跟着一个 小数点&nbsp;<code>.</code>。</li>
	<li>数字&nbsp;后跟着一个 小数点&nbsp;<code>.</code>&nbsp;再跟着 数位。</li>
	<li>一个 小数点&nbsp;<code>.</code>&nbsp;后跟着 数位。</li>
</ol>

指数 定义为指数符号 <code>'e'</code> 或 <code>'E'</code>，后面跟着一个 整数。

数字&nbsp;定义为一个或多个数位。

&nbsp;

示例 1：

<div class="example-block">
输入：s = "0"

输出：true
</div>

示例 2：

<div class="example-block">
输入：s = "e"

输出：false
</div>

示例 3：

<div class="example-block">
输入：s = "."

输出：false
</div>

&nbsp;

提示：

<ul>
	<li><code>1 &lt;= s.length &lt;= 20</code></li>
	<li><code>s</code> 仅含英文字母（大写和小写），数字（<code>0-9</code>），加号 <code>'+'</code> ，减号 <code>'-'</code> ，或者点 <code>'.'</code> 。</li>
</ul>