国外版

算法题（国外版）/peak-index-in-a-mountain-array.html

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+<p>Let&#39;s call an array <code>arr</code> a <strong>mountain</strong>&nbsp;if the following properties hold:</p>
+
+<ul>
+	<li><code>arr.length &gt;= 3</code></li>
+	<li>There exists some <code>i</code> with&nbsp;<code>0 &lt; i&nbsp;&lt; arr.length - 1</code>&nbsp;such that:
+	<ul>
+		<li><code>arr[0] &lt; arr[1] &lt; ... arr[i-1] &lt; arr[i] </code></li>
+		<li><code>arr[i] &gt; arr[i+1] &gt; ... &gt; arr[arr.length - 1]</code></li>
+	</ul>
+	</li>
+</ul>
+
+<p>Given an integer array <code>arr</code> that is <strong>guaranteed</strong> to be&nbsp;a mountain, return any&nbsp;<code>i</code>&nbsp;such that&nbsp;<code>arr[0] &lt; arr[1] &lt; ... arr[i - 1] &lt; arr[i] &gt; arr[i + 1] &gt; ... &gt; arr[arr.length - 1]</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> arr = [0,1,0]
+<strong>Output:</strong> 1
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> arr = [0,2,1,0]
+<strong>Output:</strong> 1
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> arr = [0,10,5,2]
+<strong>Output:</strong> 1
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>3 &lt;= arr.length &lt;= 10<sup>4</sup></code></li>
+	<li><code>0 &lt;= arr[i] &lt;= 10<sup>6</sup></code></li>
+	<li><code>arr</code> is <strong>guaranteed</strong> to be a mountain array.</li>
+</ul>
+
+<p>&nbsp;</p>
+<strong>Follow up:</strong> Finding the <code>O(n)</code> is straightforward, could you find an <code>O(log(n))</code> solution?