<p>Given an encoded string, return its decoded string.</p>

<p>The encoding rule is: <code>k[encoded_string]</code>, where the <code>encoded_string</code> inside the square brackets is being repeated exactly <code>k</code> times. Note that <code>k</code> is guaranteed to be a positive integer.</p>

<p>You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, <code>k</code>. For example, there will not be input like <code>3a</code> or <code>2[4]</code>.</p>

<p>The test cases are generated so that the length of the output will never exceed <code>10<sup>5</sup></code>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;3[a]2[bc]&quot;
<strong>Output:</strong> &quot;aaabcbc&quot;
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;3[a2[c]]&quot;
<strong>Output:</strong> &quot;accaccacc&quot;
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;2[abc]3[cd]ef&quot;
<strong>Output:</strong> &quot;abcabccdcdcdef&quot;
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 30</code></li>
	<li><code>s</code> consists of lowercase English letters, digits, and square brackets <code>&#39;[]&#39;</code>.</li>
	<li><code>s</code> is guaranteed to be <strong>a valid</strong> input.</li>
	<li>All the integers in <code>s</code> are in the range <code>[1, 300]</code>.</li>
</ul>

/*

3[a2[c]]

upon finding `<digit>[,`
continue reading and record `<inner>]`

run decode on the inner part
output += <digit> * innerdecoded

*/

class Solution {
public:
    std::string decodeString(std::string s) {
        return dsr(s.cbegin(), s.cend());
    }
    std::string dsr(auto begin, auto end) {
        std::string decoded{};
        for (; begin != end; begin++) {
            cout << "processing: " << *begin << '\n';
            if (*begin >= '0' && *begin <= '9') {
                /* found a number, collect all the digits */
                string number{*begin};
                begin++;
                while (*begin >= '0' && *begin <= '9')
                    number.push_back(*(begin++));
                int repetitions{std::stoi(number)};
                /* now we are at the opening [ */
                auto openBracket{begin};
                int level{1};
                do {
                    begin++;
                    if (*begin == '[')
                        level++;
                    else if (*begin == ']')
                        level--;
                } while (level > 0);
                /* now we are at the closing ] */
                for (int i{0}; i < repetitions; i++) {
                    decoded += dsr(openBracket + 1, begin);
                }
            } else {
                /* regular characters just pass through */
                decoded.push_back(*begin);
            }
        }
        return decoded;
    }
};
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