852. Peak Index in a Mountain Array

Author: remy727

README.md

@@ -234,6 +234,7 @@
 | 839 | Similar String Groups | [Ruby](./algorithms/ruby/0839-similar-string-groups.rb) | Hard |
 | 841 | Backspace String Compare | [Ruby](./algorithms/ruby/0841-keys-and-rooms.rb) | Medium |
 | 844 | Backspace String Compare | [Ruby](./algorithms/ruby/0844-backspace-string-compare.rb) | Easy |
+| 852 | Peak Index in a Mountain Array | [Ruby](./algorithms/ruby/0852-peak-index-in-a-mountain-array.rb) | Medium |
 | 859 | Buddy Strings | [Ruby](./algorithms/ruby/0859-buddy-strings.rb) | Easy |
 | 863 | All Nodes Distance K in Binary Tree | [Ruby](./algorithms/ruby/0863-all-nodes-distance-k-in-binary-tree.rb) | Medium |
 | 864 | Backspace String Compare | [Ruby](./algorithms/ruby/0864-shortest-path-to-get-all-keys.rb) | Hard |

algorithms/ruby/0852-peak-index-in-a-mountain-array.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+# 852. Peak Index in a Mountain Array
+# Medium
+# https://leetcode.com/problems/peak-index-in-a-mountain-array
+
+=begin
+An array arr a mountain if the following properties hold:
+* arr.length >= 3
+* There exists some i with 0 < i < arr.length - 1 such that:
+  * arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
+  * arr[i] > arr[i + 1] > ... > arr[arr.length - 1]
+Given a mountain array arr, return the index i such that arr[0] < arr[1] < ... < arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].
+You must solve it in O(log(arr.length)) time complexity.
+
+Example 1:
+Input: arr = [0,1,0]
+Output: 1
+
+Example 2:
+Input: arr = [0,2,1,0]
+Output: 1
+
+Example 3:
+Input: arr = [0,10,5,2]
+Output: 1
+
+Constraints:
+* 3 <= arr.length <= 105
+* 0 <= arr[i] <= 106
+* arr is guaranteed to be a mountain array.
+=end
+
+# @param {Integer[]} arr
+# @return {Integer}
+def peak_index_in_mountain_array(arr)
+  low = 0
+  high = arr.length - 1
+
+  while low < high
+    mid = low + (high - low) / 2
+
+    if arr[mid] < arr[mid + 1]
+      low = mid + 1
+    else
+      high = mid
+    end
+  end
+
+  low
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_peak_index_in_mountain_array < Test::Unit::TestCase
+  def test_
+    assert_equal 1, peak_index_in_mountain_array([0, 1, 0])
+    assert_equal 1, peak_index_in_mountain_array([0, 2, 1, 0])
+    assert_equal 1, peak_index_in_mountain_array([0, 10, 5, 2])
+  end
+end