<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) {
        if(digits.empty()){ return {};}
        vector<string> res = phonePad("", digits);
        return res;
       
        
    }
    public:
        vector<string>phonePad(string p, string up){
        vector<string> ans;
        if(up.empty()){
            ans.push_back(p);
            return ans;
        }
        int digits =up[0]-'0';
        int start = (digits-2)*3;
        int end = (digits-1)*3;
        if(digits == 8 ){
            start = start+1;
            end = end+1;
        }
        if(digits == 7){
            end = end+1;
        }
        if(digits == 9){
            start = start+1;
            end = end+2;
        }


        for(int i = start; i<end; i++){
           char ch = ('a'+i);
           vector<string> comb = phonePad(p+ch, up.substr(1));
           ans.insert(ans.end(), comb.begin(), comb.end());
        }
        return ans;

        }
};
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