<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>

class Solution {
private:
    string s;
    unordered_map<int, int> leftToRight, rightToLeft;

    void processBrackets() {
        stack<int> stk;
        for (int i = 0; i < s.length(); i++) {
            if (s[i] == '[') stk.push(i);
            else if (s[i] == ']') {
                leftToRight[stk.top()] = i;
                rightToLeft[i] = stk.top();
                stk.pop();
            }
        } 
    }

    string repeat(int num, string str) {
        return num == 0 ? "" : str + repeat(num - 1, str);
    }

    string decodeUtil(int fromIdx, int toIdx) {
        string result = "", num = "";
        int i = fromIdx;
        while (i <= toIdx) {
            if (s[i] >= 'a' && s[i] <= 'z') result += s[i++];
            else if (s[i] >= '0' && s[i] <= '9') num += s[i++];
            else { // s[i] == '['
                result += repeat(stoi(num), decodeUtil(i + 1, leftToRight[i] - 1));
                i = leftToRight[i] + 1;
                num = "";
            }
        }
        return result;
    }

public:
    string decodeString(string s) {
        this->s = s;
        processBrackets();
        return decodeUtil(0, s.length() - 1);
    }
};
9100