String to Integer (atoi)
<p>Implement the <code>myAtoi(string s)</code> function, which converts a string to a 32-bit signed integer.</p>

<p>The algorithm for <code>myAtoi(string s)</code> is as follows:</p>

<ol>
	<li><strong>Whitespace</strong>: Ignore any leading whitespace (<code>&quot; &quot;</code>).</li>
	<li><strong>Signedness</strong>: Determine the sign by checking if the next character is <code>&#39;-&#39;</code> or <code>&#39;+&#39;</code>, assuming positivity if neither present.</li>
	<li><strong>Conversion</strong>: Read the integer by skipping leading zeros&nbsp;until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.</li>
	<li><strong>Rounding</strong>: If the integer is out of the 32-bit signed integer range <code>[-2<sup>31</sup>, 2<sup>31</sup> - 1]</code>, then round the integer to remain in the range. Specifically, integers less than <code>-2<sup>31</sup></code> should be rounded to <code>-2<sup>31</sup></code>, and integers greater than <code>2<sup>31</sup> - 1</code> should be rounded to <code>2<sup>31</sup> - 1</code>.</li>
</ol>

<p>Return the integer as the final result.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;42&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">42</span></p>

<p><strong>Explanation:</strong></p>

<pre>
The underlined characters are what is read in and the caret is the current reader position.
Step 1: &quot;42&quot; (no characters read because there is no leading whitespace)
         ^
Step 2: &quot;42&quot; (no characters read because there is neither a &#39;-&#39; nor &#39;+&#39;)
         ^
Step 3: &quot;<u>42</u>&quot; (&quot;42&quot; is read in)
           ^
</pre>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot; -042&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">-42</span></p>

<p><strong>Explanation:</strong></p>

<pre>
Step 1: &quot;<u>   </u>-042&quot; (leading whitespace is read and ignored)
            ^
Step 2: &quot;   <u>-</u>042&quot; (&#39;-&#39; is read, so the result should be negative)
             ^
Step 3: &quot;   -<u>042</u>&quot; (&quot;042&quot; is read in, leading zeros ignored in the result)
               ^
</pre>
</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;1337c0d3&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">1337</span></p>

<p><strong>Explanation:</strong></p>

<pre>
Step 1: &quot;1337c0d3&quot; (no characters read because there is no leading whitespace)
         ^
Step 2: &quot;1337c0d3&quot; (no characters read because there is neither a &#39;-&#39; nor &#39;+&#39;)
         ^
Step 3: &quot;<u>1337</u>c0d3&quot; (&quot;1337&quot; is read in; reading stops because the next character is a non-digit)
             ^
</pre>
</div>

<p><strong class="example">Example 4:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;0-1&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">0</span></p>

<p><strong>Explanation:</strong></p>

<pre>
Step 1: &quot;0-1&quot; (no characters read because there is no leading whitespace)
         ^
Step 2: &quot;0-1&quot; (no characters read because there is neither a &#39;-&#39; nor &#39;+&#39;)
         ^
Step 3: &quot;<u>0</u>-1&quot; (&quot;0&quot; is read in; reading stops because the next character is a non-digit)
          ^
</pre>
</div>

<p><strong class="example">Example 5:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;words and 987&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">0</span></p>

<p><strong>Explanation:</strong></p>

<p>Reading stops at the first non-digit character &#39;w&#39;.</p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= s.length &lt;= 200</code></li>
	<li><code>s</code> consists of English letters (lower-case and upper-case), digits (<code>0-9</code>), <code>&#39; &#39;</code>, <code>&#39;+&#39;</code>, <code>&#39;-&#39;</code>, and <code>&#39;.&#39;</code>.</li>
</ul>
String
class Solution:
    def myAtoi(self, s: str) -> int:
        if not s:
            return 0
        n = len(s)
        if n == 0:
            return 0
        i = 0
        while s[i] == ' ':
            i += 1
            # 仅包含空格
            if i == n:
                return 0
        sign = -1 if s[i] == '-' else 1
        if s[i] in ['-', '+']:
            i += 1
        res, flag = 0, (2**31 - 1) // 10
        while i < n:
            # 非数字，跳出循环体
            if not s[i].isdigit():
                break
            c = int(s[i])
            # 溢出判断
            if res > flag or (res == flag and c > 7):
                return 2**31 - 1 if sign > 0 else -(2**31)
            res = res * 10 + c
            i += 1
        return sign * res
