<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:
    bool isDigit(string ch) {
        if(ch == "0" || ch == "1" || ch == "2" || ch == "3" || ch == "4" || ch == "5"
          || ch == "6" || ch == "7" || ch == "8" || ch == "9")
            return true;
        return false;
    }
    string decodeString(string s) {
        stack<string> st;
        
        for(char ch:s) {
            if(ch != ']') {
                st.push(string(1, ch));
                continue;
            }
            
            string temp = "";
            while(!st.empty() && st.top() != "[") {
                temp = st.top() + temp;
                st.pop();
            }
            
            if(!st.empty()) { // to remove "["
                st.pop();
            }
            
            string num = "";
            while(!st.empty() && isDigit(st.top())) {
                num = st.top() + num;
                st.pop();
            }
            
            string add = "";
            int times = stoi(num);
            while(times--) {
                add += temp;
            }
            
            st.push(add);
        }
        string result = "";
        while(!st.empty()) {
            result = st.top() + result;
            st.pop();
        }
        return result;
    }
};
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