<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 {
public:
    string decodeString(string s) {
        stack<int> num; 
        stack<string> ch;  // Changed to stack<string> to handle nested strings
        string decoded = "";
        int count = 0;  // To handle numbers greater than 9

        for (int i = 0; i < s.size(); i++) {
            if (isdigit(s[i])) {
                count = count * 10 + (s[i] - '0');  // Build full number
            } 
            else if (s[i] == '[') {
                num.push(count);  // Push full number to stack
                count = 0;  // Reset count
                ch.push(decoded);  // Push current decoded string to stack
                decoded = "";  // Reset decoded for new substring
            } 
            else if (s[i] == ']') {
                string ans = decoded;  // The substring inside brackets
                decoded = ch.top();  // Get previous decoded string
                ch.pop();

                int repeat = num.top();  // Get repeat count
                num.pop();

                for (int j = 0; j < repeat; j++) {
                    decoded += ans;  // Repeat the substring
                }
            } 
            else {
                decoded += s[i];  // Append characters to the current string
            }
        }

        return decoded;
    }
};

7900