add leetcode problem-cn part4

算法题（国内版）/problem (English)/交替合并字符串(English) [merge-strings-alternately].html

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+<p>You are given two strings <code>word1</code> and <code>word2</code>. Merge the strings by adding letters in alternating order, starting with <code>word1</code>. If a string is longer than the other, append the additional letters onto the end of the merged string.</p>
+
+<p>Return <em>the merged string.</em></p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> word1 = &quot;abc&quot;, word2 = &quot;pqr&quot;
+<strong>Output:</strong> &quot;apbqcr&quot;
+<strong>Explanation:</strong>&nbsp;The merged string will be merged as so:
+word1:  a   b   c
+word2:    p   q   r
+merged: a p b q c r
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> word1 = &quot;ab&quot;, word2 = &quot;pqrs&quot;
+<strong>Output:</strong> &quot;apbqrs&quot;
+<strong>Explanation:</strong>&nbsp;Notice that as word2 is longer, &quot;rs&quot; is appended to the end.
+word1:  a   b 
+word2:    p   q   r   s
+merged: a p b q   r   s
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> word1 = &quot;abcd&quot;, word2 = &quot;pq&quot;
+<strong>Output:</strong> &quot;apbqcd&quot;
+<strong>Explanation:</strong>&nbsp;Notice that as word1 is longer, &quot;cd&quot; is appended to the end.
+word1:  a   b   c   d
+word2:    p   q 
+merged: a p b q c   d
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= word1.length, word2.length &lt;= 100</code></li>
+	<li><code>word1</code> and <code>word2</code> consist of lowercase English letters.</li>
+</ul>