<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> letterCombinations(string digits) {
        vector<string> output = {""};
        for(char dig: digits){
            vector<char> toAdd;
            switch(dig){
                case '2':
                    toAdd = {'a','b','c'};
                    break;
                case '3':
                    toAdd = {'d','e','f'};
                    break;
                case '4':
                    toAdd = {'g','h','i'};
                    break;
                case '5':
                    toAdd = {'j','k','l'};
                    break;
                case '6':
                    toAdd = {'m','n','o'};
                    break;
                case '7':
                    toAdd = {'p','q','r','s'};
                    break;
                case '8':
                    toAdd = {'t','u','v'};
                    break;
                case '9':
                    toAdd = {'w','x','y','z'};
                    break;  
            }
            int m = toAdd.size();
            if(m > 0){
                int n = output.size();
                for(int i = 0; i < n; i++){
                    string temp = output[i];
                    output[i] += toAdd[0];
                    for(int j = 1; j < m; j++){
                        output.push_back(temp+toAdd[j]);
                    }
                }
            }
        }
        if(output[0]== ""){return {};}
        return output;
    }
};
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