<p>Given an input string <code>s</code>, reverse the order of the <strong>words</strong>.</p>

<p>A <strong>word</strong> is defined as a sequence of non-space characters. The <strong>words</strong> in <code>s</code> will be separated by at least one space.</p>

<p>Return <em>a string of the words in reverse order concatenated by a single space.</em></p>

<p><b>Note</b> that <code>s</code> may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.</p>

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

<pre>
<strong>Input:</strong> s = &quot;the sky is blue&quot;
<strong>Output:</strong> &quot;blue is sky the&quot;
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;  hello world  &quot;
<strong>Output:</strong> &quot;world hello&quot;
<strong>Explanation:</strong> Your reversed string should not contain leading or trailing spaces.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;a good   example&quot;
<strong>Output:</strong> &quot;example good a&quot;
<strong>Explanation:</strong> You need to reduce multiple spaces between two words to a single space in the reversed string.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>4</sup></code></li>
	<li><code>s</code> contains English letters (upper-case and lower-case), digits, and spaces <code>&#39; &#39;</code>.</li>
	<li>There is <strong>at least one</strong> word in <code>s</code>.</li>
</ul>

<p>&nbsp;</p>
<p><b data-stringify-type="bold">Follow-up:&nbsp;</b>If the string data type is mutable in your language, can&nbsp;you solve it&nbsp;<b data-stringify-type="bold">in-place</b>&nbsp;with&nbsp;<code data-stringify-type="code">O(1)</code>&nbsp;extra space?</p>

class Solution {
public:
    string reverseWords(string s) {
        int length = s.length();
        
        // removes leading spaces
        int index = 0;
        while(index < length && s[index] == ' ') index++;
        
        int avail = 0;
        for(int i = index; i < length; i++)
        {
            // If current character is a non space character,
            // or a space but without trailing space character.
            // Checking if there's still a next char, before checking if it is a space char.
            if(s[i] != ' ' || (s[i] == ' ' && (i + 1 < length && s[i + 1] != ' '))) 
            {
                s[avail++] = s[i];
            }
        }
        
        // Avail is like the validated string's length.
        length = avail;
        s = s.substr(0, length);
        
        
        // Reversing the whole string.
        s = reverse(s, 0, length - 1);
        
        // Reversing-back each word of the string.
        // Variable start will store the index of the first letter of each word.
        int start = 0;
        for(int i = 0; i < length; i++)
        {
            while(i < length && s[i] != ' ') i++;
            s = reverse(s, start, i - 1);
            start = i + 1;
        }
        
        return s;
    }
    
    string reverse(string s, int start, int end)
    {
        while(start < end)
        {
            char temp = s[start];
            s[start++] = s[end];
            s[end--] = temp;
        }
        
        return s;
    }
};
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