<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 RadixTree {
    std::vector<std::unique_ptr<RadixTree>> children;
    int count = 0;
    int used = 0;

public:
    void add(string_view&& w) {
        if (w.empty()) {
            ++count;
            return;
        }

        children.resize('z' - 'a' + 1);
        auto& child = children[w[0] - 'a'];
        if (!child)
            child = make_unique<RadixTree>();

        w.remove_prefix(1);
        child->add(std::move(w));
    }

    bool use(string_view&& w) {
        if (used == -1) {
            // Push pending reset onto children
            used = 0;
            for (auto& child: children)
                if (child)
                    child->reset();
        }

        if (w.empty()) {
            return ++used <= count;
        }

        children.resize('z' - 'a' + 1);
        auto& child = children[w[0] - 'a'];
        if (!child)
            child = make_unique<RadixTree>();

        w.remove_prefix(1);
        return child->use(std::move(w));
    }

    void reset() {
        used = -1;
    }
};

class Solution {
public:
    vector<int> findSubstring(string s, vector<string>& words) {
        RadixTree tree;
        for (auto const& w: words) {
            tree.add(w);
        }

        int wlen = words[0].length();
        int tlen = words.size() * wlen;

        std::vector<int> result;
        for (int i = 0, last = int(s.length()) - tlen; i <= last; ++i)
            if (match(tree, wlen, std::string_view(s).substr(i, tlen)))
                result.push_back(i);

        return result;
    }

private:
    bool match(RadixTree& tree, size_t wlen, std::string_view s) {
        bool result = true;

        for (int i = 0; i < s.length(); i += wlen) {
            if (!tree.use(s.substr(i, wlen))) {
                result = false;
                break;
            }
        }

        tree.reset();
        return result;
    }
};

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