string-to-integer-atoi.md

<p>Implement <code><span>atoi</span></code> which&nbsp;converts a string to an integer.</p>

<p>The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.</p>

<p>The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.</p>

<p>If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.</p>

<p>If no valid conversion could be performed, a zero value is returned.</p>

<p><strong>Note:</strong></p>

<ul>
	<li>Only the space character <code>&#39; &#39;</code> is considered as whitespace character.</li>
	<li>Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [&minus;2<sup>31</sup>,&nbsp; 2<sup>31&nbsp;</sup>&minus; 1]. If the numerical value is out of the range of representable values, INT_MAX (2<sup>31&nbsp;</sup>&minus; 1) or INT_MIN (&minus;2<sup>31</sup>) is returned.</li>
</ul>

<p><strong>Example 1:</strong></p>

<pre>
<strong>Input:</strong> &quot;42&quot;
<strong>Output:</strong> 42
</pre>

<p><strong>Example 2:</strong></p>

<pre>
<strong>Input:</strong> &quot;   -42&quot;
<strong>Output:</strong> -42
<strong>Explanation:</strong> The first non-whitespace character is &#39;-&#39;, which is the minus sign.
&nbsp;            Then take as many numerical digits as possible, which gets 42.
</pre>

<p><strong>Example 3:</strong></p>

<pre>
<strong>Input:</strong> &quot;4193 with words&quot;
<strong>Output:</strong> 4193
<strong>Explanation:</strong> Conversion stops at digit &#39;3&#39; as the next character is not a numerical digit.
</pre>

<p><strong>Example 4:</strong></p>

<pre>
<strong>Input:</strong> &quot;words and 987&quot;
<strong>Output:</strong> 0
<strong>Explanation:</strong> The first non-whitespace character is &#39;w&#39;, which is not a numerical 
&nbsp;            digit or a +/- sign. Therefore no valid conversion could be performed.</pre>

<p><strong>Example 5:</strong></p>

<pre>
<strong>Input:</strong> &quot;-91283472332&quot;
<strong>Output:</strong> -2147483648
<strong>Explanation:</strong> The number &quot;-91283472332&quot; is out of the range of a 32-bit signed integer.
&nbsp;            Thefore INT_MIN (&minus;2<sup>31</sup>) is returned.</pre>