<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 convert(string n,string s){
        string temp="";
        for(int i=0;i<stoi(n);i++){
            temp+=s;
        }
        return temp;
    }
    string decodeString(string s) {
        // using recursion : 
        // int i=0;
        // return helper(i,s);

        // using most optimal approach : stack 
        stack<int>st;
        string ans="";
        for(int i=0;i<s.size();i++){
            if(s[i]!=']'){
                st.push(s[i]);
            }
            else{
                string temp="";
                string n="";
                while(!st.empty() && !isdigit(st.top())){
                    if(st.top()=='[') {
                        st.pop();
                        continue;
                    }
                    temp+=string(1,st.top());
                    st.pop();
                }                
                while(!st.empty() && isdigit(st.top())){
                    n+=string(1,st.top());
                    st.pop();
                }
                reverse(temp.begin(),temp.end());
                reverse(n.begin(),n.end());
                string z=convert(n,temp);   
                for(auto x:z){
                    st.push(x);
                }          
            }
        }
        while(!st.empty()){
            ans+=string(1,st.top());
            st.pop();
        }
        reverse(ans.begin(),ans.end());
        return ans;
    }
    string helper(int &idx,string s){
        int n=0;
        string ans="";
        while(idx<s.size()){
            char c=s[idx];
            if(c>='0' && c<='9') n=n*10 +c-'0';
            else if(c=='['){
                string temp=helper(++idx,s);
                while(n>0){
                    ans+=temp;
                    n--;
                }
            }
            else if(c==']') return ans;
            else ans+=c;
            idx++;
        }
        return ans;
    }
};
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