<p>You are given a string <code>s</code> and an array of strings <code>words</code>. All the strings of <code>words</code> are of <strong>the same length</strong>.</p>

<p>A <strong>concatenated string</strong> is a string that exactly contains all the strings of any permutation of <code>words</code> concatenated.</p>

<ul>
	<li>For example, if <code>words = [&quot;ab&quot;,&quot;cd&quot;,&quot;ef&quot;]</code>, then <code>&quot;abcdef&quot;</code>, <code>&quot;abefcd&quot;</code>, <code>&quot;cdabef&quot;</code>, <code>&quot;cdefab&quot;</code>, <code>&quot;efabcd&quot;</code>, and <code>&quot;efcdab&quot;</code> are all concatenated strings. <code>&quot;acdbef&quot;</code> is not a concatenated string because it is not the concatenation of any permutation of <code>words</code>.</li>
</ul>

<p>Return an array of <em>the starting indices</em> of all the concatenated substrings in <code>s</code>. You can return the answer in <strong>any order</strong>.</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;barfoothefoobarman&quot;, words = [&quot;foo&quot;,&quot;bar&quot;]</span></p>

<p><strong>Output:</strong> <span class="example-io">[0,9]</span></p>

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

<p>The substring starting at 0 is <code>&quot;barfoo&quot;</code>. It is the concatenation of <code>[&quot;bar&quot;,&quot;foo&quot;]</code> which is a permutation of <code>words</code>.<br />
The substring starting at 9 is <code>&quot;foobar&quot;</code>. It is the concatenation of <code>[&quot;foo&quot;,&quot;bar&quot;]</code> which is a permutation of <code>words</code>.</p>
</div>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;wordgoodgoodgoodbestword&quot;, words = [&quot;word&quot;,&quot;good&quot;,&quot;best&quot;,&quot;word&quot;]</span></p>

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

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

<p>There is no concatenated substring.</p>
</div>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;barfoofoobarthefoobarman&quot;, words = [&quot;bar&quot;,&quot;foo&quot;,&quot;the&quot;]</span></p>

<p><strong>Output:</strong> <span class="example-io">[6,9,12]</span></p>

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

<p>The substring starting at 6 is <code>&quot;foobarthe&quot;</code>. It is the concatenation of <code>[&quot;foo&quot;,&quot;bar&quot;,&quot;the&quot;]</code>.<br />
The substring starting at 9 is <code>&quot;barthefoo&quot;</code>. It is the concatenation of <code>[&quot;bar&quot;,&quot;the&quot;,&quot;foo&quot;]</code>.<br />
The substring starting at 12 is <code>&quot;thefoobar&quot;</code>. It is the concatenation of <code>[&quot;the&quot;,&quot;foo&quot;,&quot;bar&quot;]</code>.</p>
</div>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>4</sup></code></li>
	<li><code>1 &lt;= words.length &lt;= 5000</code></li>
	<li><code>1 &lt;= words[i].length &lt;= 30</code></li>
	<li><code>s</code> and <code>words[i]</code> consist of lowercase English letters.</li>
</ul>

class Solution {
public:

    bool isCandidate( const string& _str, const string& _refStr)
    {
        return _str == _refStr;
    }
    
    bool isCandidate( const string& _str, unordered_map<string, int>& wordMap )
    {
        unordered_map<string, int> seen;

        int i = 0;
        while ( i <= _str.size() - m_wordLen )
        {
            const string& aStr = _str.substr(i, m_wordLen);
            if ( wordMap.find(aStr) == wordMap.end() )
            {
                return false;
            }

            if ( seen.find(aStr) == seen.end() )
            {
                seen[aStr] = 1;
            }
            else
            {
                seen[aStr]++;
            }

            if ( seen[aStr] > wordMap[aStr] )
            {
                return false;
            }
            i += m_wordLen;
        }

        return true;
    }

    vector<int> findSubstring(string s, vector<string>& words) 
    {
        m_stringLen = words.size() * words[0].size();
        m_wordLen = words[0].size();
        if ( m_stringLen > s.size() ) return {};

        unordered_map<string, int> wordMap;
        for (int i = 0; i < words.size(); i++ )
        {
            if ( wordMap.find( words[i] ) == wordMap.end() )
            {
                wordMap[words[i]] = 1;
            }
            else 
            {
                wordMap[words[i]] += 1;
            }
        }

        string _refStr;
        if ( wordMap.size() == 1 && words[0].size() == 1 )
        {
            const char& _c = words[0][0];
            _refStr = string( words.size(), _c );
        }

        vector<int> ans;
        
        if ( _refStr.size() == 0 )
        {
            for ( int i = 0; i <= s.size() - m_stringLen; i++ )
            {
                if ( isCandidate( s.substr(i, m_stringLen), wordMap ) )
                {
                    ans.push_back( i );
                }
            }
        }
        else
        {
            for ( int i = 0; i <= s.size() - m_stringLen; i++ )
            {
                if ( isCandidate( s.substr(i, m_stringLen), _refStr ) )
                {
                    ans.push_back( i );
                }
            }
        }

        return ans;
    }

private:
    unordered_set<string> m_candidates;
    int m_wordLen;
    int m_stringLen;
};
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