<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 solve(int ind,vector<string>st,vector<string>&ans,string res){
        
        if(ind == st.size()){
            if(res!="")
             ans.push_back(res);
            return;
        }
        
        for(int i = 0;i<st[ind].size();i++){
              res+=st[ind][i];
              solve(ind+1,st,ans,res);
              res.pop_back();
             
        }
         
    }
    vector<string> letterCombinations(string digits) {
       
         
        unordered_map<int,string>mp;
        
        mp[2] = "abc";
        mp[3] = "def";
        mp[4] = "ghi";
        mp[5] = "jkl";
        mp[6] = "mno";
        mp[7] = "pqrs";
        mp[8] = "tuv";
        mp[9] = "wxyz";
        
        vector<string>st;
        
        for(int i = 0;i<digits.size();i++){
            st.push_back(mp[digits[i] - '0']);
        }
        vector<string>ans;
        
        solve(0,st,ans,"");
        return ans;
        
    }
};
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