<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> mapping(char c){
        if(c=='2') return {"a", "b", "c"};
        else if(c=='3') return {"d", "e", "f"};
        else if(c=='4') return {"g", "h", "i"};
        else if(c=='5') return {"j", "k", "l"};
        else if(c=='6') return {"m", "n", "o"};
        else if(c=='7') return {"p", "q", "r","s"};
        else if(c=='8') return {"t", "u", "v"};
        else if(c=='9') return {"w", "x", "y", "z"};
        else return {};
    }
    void helper(int i, string temp, vector<string> &res, string digits){
        if(i==digits.size())res.push_back(temp);
        else{
            vector<string> map = mapping(digits[i]);
            for(int j=0; j<map.size(); j++){
                temp.push_back(map[j].front());
                helper(i+1, temp, res, digits);
                temp.pop_back();
            }
        }
        return;
    }
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if(digits=="") return res;
        helper(0, "", res, digits);
        return res;
    }
};
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