Peak Index in a Mountain Array
<p>You are given an integer <strong>mountain</strong> array <code>arr</code> of length <code>n</code> where the values increase to a <strong>peak element</strong> and then decrease.</p>

<p>Return the index of the peak element.</p>

<p>Your task is to solve it in <code>O(log(n))</code> time complexity.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">arr = [0,1,0]</span></p>

<p><strong>Output:</strong> <span class="example-io">1</span></p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">arr = [0,2,1,0]</span></p>

<p><strong>Output:</strong> <span class="example-io">1</span></p>
</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">arr = [0,10,5,2]</span></p>

<p><strong>Output:</strong> <span class="example-io">1</span></p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>3 &lt;= arr.length &lt;= 10<sup>5</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>
Array; Binary Search
impl Solution {
    pub fn peak_index_in_mountain_array(arr: Vec<i32>) -> i32 {
        let mut left = 1;
        let mut right = arr.len() - 2;
        while left < right {
            let mid = left + (right - left) / 2;
            if arr[mid] > arr[mid + 1] {
                right = mid;
            } else {
                left = left + 1;
            }
        }
        left as i32
    }
}
