<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:

    void recursion(string digits, vector<string> mappings, int index, string & temp, vector<string> & ans) {

        if(index == digits.size()) {
            ans.push_back(temp);
            return;
        }

        string letters = mappings[digits[index] - '2'];

        for(char letter : letters) {
            temp.push_back(letter);
            recursion(digits, mappings, index+1, temp, ans);
            temp.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        if(digits.size() == 0) {
            return {};
        }
        vector<string> ans;

        vector<string> mappings = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

        int index = 0;
        string temp = "";
        recursion(digits, mappings, index, temp, ans);

        return ans;
    }
};

/*

recursion(digits, index, vector<string> mapp, temp, ans)

if(digits size == index) {
    ans.push (temp);
    return;
}
index = 0
string letters = "abc"
for (char : letters)
temp+=char;
recursion(digits, index+1, mapp, )
temp.pop_back();

3^n == output size

n : 

temp string = "ad";

23
3 : def
2 : abc
ad
*/
9600