add leetcode problem-cn part6

算法题（国内版）/problem (Chinese)/电话号码的字母组合 [letter-combinations-of-a-phone-number].html

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+<p>给定一个仅包含数字&nbsp;<code>2-9</code>&nbsp;的字符串，返回所有它能表示的字母组合。答案可以按 <strong>任意顺序</strong> 返回。</p>
+
+<p>给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。</p>
+
+<p><img src="https://assets.leetcode-cn.com/aliyun-lc-upload/uploads/2021/11/09/200px-telephone-keypad2svg.png" style="width: 200px;" /></p>
+
+<p>&nbsp;</p>
+
+<p><strong>示例 1：</strong></p>
+
+<pre>
+<strong>输入：</strong>digits = "23"
+<strong>输出：</strong>["ad","ae","af","bd","be","bf","cd","ce","cf"]
+</pre>
+
+<p><strong>示例 2：</strong></p>
+
+<pre>
+<strong>输入：</strong>digits = ""
+<strong>输出：</strong>[]
+</pre>
+
+<p><strong>示例 3：</strong></p>
+
+<pre>
+<strong>输入：</strong>digits = "2"
+<strong>输出：</strong>["a","b","c"]
+</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>提示：</strong></p>
+
+<ul>
+	<li><code>0 &lt;= digits.length &lt;= 4</code></li>
+	<li><code>digits[i]</code> 是范围 <code>['2', '9']</code> 的一个数字。</li>
+</ul>