add leetcode problem-cn part2

算法题（国内版）/problem (English)/检查是否所有 A 都在 B 之前(English) [check-if-all-as-appears-before-all-bs].html

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+<p>Given a string <code>s</code> consisting of <strong>only</strong> the characters <code>&#39;a&#39;</code> and <code>&#39;b&#39;</code>, return <code>true</code> <em>if <strong>every</strong> </em><code>&#39;a&#39;</code> <em>appears before <strong>every</strong> </em><code>&#39;b&#39;</code><em> in the string</em>. Otherwise, return <code>false</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;aaabbb&quot;
+<strong>Output:</strong> true
+<strong>Explanation:</strong>
+The &#39;a&#39;s are at indices 0, 1, and 2, while the &#39;b&#39;s are at indices 3, 4, and 5.
+Hence, every &#39;a&#39; appears before every &#39;b&#39; and we return true.
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;abab&quot;
+<strong>Output:</strong> false
+<strong>Explanation:</strong>
+There is an &#39;a&#39; at index 2 and a &#39;b&#39; at index 1.
+Hence, not every &#39;a&#39; appears before every &#39;b&#39; and we return false.
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;bbb&quot;
+<strong>Output:</strong> true
+<strong>Explanation:</strong>
+There are no &#39;a&#39;s, hence, every &#39;a&#39; appears before every &#39;b&#39; and we return true.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= s.length &lt;= 100</code></li>
+	<li><code>s[i]</code> is either <code>&#39;a&#39;</code> or <code>&#39;b&#39;</code>.</li>
+</ul>