<p>Given a string containing digits from <code>2-9</code> inclusive, return all possible letter combinations that the number could represent. Return the answer in <strong>any order</strong>.</p>

<p>A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.</p>
<img alt="" src="https://assets.leetcode.com/uploads/2022/03/15/1200px-telephone-keypad2svg.png" style="width: 300px; height: 243px;" />
<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> digits = &quot;23&quot;
<strong>Output:</strong> [&quot;ad&quot;,&quot;ae&quot;,&quot;af&quot;,&quot;bd&quot;,&quot;be&quot;,&quot;bf&quot;,&quot;cd&quot;,&quot;ce&quot;,&quot;cf&quot;]
</pre>

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

<pre>
<strong>Input:</strong> digits = &quot;&quot;
<strong>Output:</strong> []
</pre>

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

<pre>
<strong>Input:</strong> digits = &quot;2&quot;
<strong>Output:</strong> [&quot;a&quot;,&quot;b&quot;,&quot;c&quot;]
</pre>

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

<ul>
	<li><code>0 &lt;= digits.length &lt;= 4</code></li>
	<li><code>digits[i]</code> is a digit in the range <code>[&#39;2&#39;, &#39;9&#39;]</code>.</li>
</ul>

class Solution {
public:

    vector<string> progressive_ans_builder(vector<string> s1, string s2) {
        vector<string> s3; int n_s2 = s2.length();
        for(int i = 0; i<n_s2; i++) {
            for(int j = 0; j<s1.size(); j++) {
                string t = s1[j]+s2[i];
                s3.push_back(t);
            }
        }
        return s3;
    }

    vector<string> letterCombinations(string digits) {
        unordered_map<char, string> mp {
            {'2', "abc"}, {'3', "def"}, {'4', "ghi"},
            {'5', "jkl"}, {'6', "mno"}, {'7', "pqrs"},
            {'8', "tuv"}, {'9', "wxyz"}
        };

        if(digits.length()==0) return {};

        vector<string> s3 = {""};
        for(int i = 0; i<digits.length(); i++) {
            string temp = mp[digits[i]];
            s3 = progressive_ans_builder(s3, temp);
        }

        return s3;

    }
};
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