Add 852. Peak Index in a Mountain Array

Author: sierikov

src/main/scala/sierikov/leetcode/tasks/medium/PeakIndexInMountainArray.scala

@@ -0,0 +1,34 @@
+package sierikov.leetcode.tasks.medium
+
+// leeetcode: https://leetcode.com/problems/peak-index-in-a-mountain-array/
+object PeakIndexInMountainArray {
+  def peakIndexInMountainArray(arr: Array[Int]): Int = {
+    // borders in which we are searching
+    var start = 0;
+    var end = arr.length - 1;
+
+    // here we can check if array size is minimum 3 elements
+    // in this task it is guaranted by constrains
+    // in the case if arr.length = 1, middle will be 0,
+    // and middle + 1 will fail
+
+    // binary search moves start and end to each other
+    // and cuts the range of the search in each itteration by 2
+    while (start < end) {
+      val middle = (start + end) / 2
+      val pointA = arr(middle)
+      val pointB = arr(middle + 1)
+
+      println(s"start is $start and end is $end in ${arr.mkString(",")}")
+
+      if (pointA < pointB) // increasing part
+        start = middle + 1 // continue to search "right" part
+      else
+        end = middle // continue to search "left" part
+    }
+
+    // here we must have A(start) > A(end)
+    // this means, that we have found beginning of decreasing trend
+    start
+  }
+}

src/test/scala/sierikov/leetcode/tasks/medium/PeakIndexInMountainArrayTest.scala

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+package sierikov.leetcode.tasks.medium
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class PeakIndexInMountainArrayTest extends AnyFlatSpec with Matchers {
+  import PeakIndexInMountainArray._
+
+  it should "pass example test" in {
+    peakIndexInMountainArray(Array(0, 1, 0)) shouldBe 1
+    peakIndexInMountainArray(Array(0, 2, 1, 0)) shouldBe 1
+    peakIndexInMountainArray(Array(0, 10, 5, 2)) shouldBe 1
+  }
+
+}