update difficulty

README.md

@@ -414,6 +414,7 @@
 | 1657 | Determine if Two Strings Are Close | [Ruby](./algorithms/ruby/1657-determine-if-two-strings-are-close.rb) | Medium |
 | 1658 | Minimum Operations to Reduce X to Zero | [Ruby](./algorithms/ruby/1658-minimum-operations-to-reduce-x-to-zero.rb) | Medium |
 | 1662 | Check If Two String Arrays are Equivalent | [Ruby](./algorithms/ruby/1662-check-if-two-string-arrays-are-equivalent.rb) | Easy |
+| 1671 | Minimum Number of Removals to Make Mountain Array | [Ruby](./algorithms/ruby/1671-minimum-number-of-removals-to-make-mountain-array.rb) | Hard |
 | 1675 | Minimize Deviation in Array | [Ruby](./algorithms/ruby/1675-minimize-deviation-in-array.rb) | Hard |
 | 1679 | Max Number of K-Sum Pairs | [Ruby](./algorithms/ruby/1679-max-number-of-k-sum-pairs.rb) | Medium |
 | 1680 | Concatenation of Consecutive Binary Numbers | [Ruby](./algorithms/ruby/1680-concatenation-of-consecutive-binary-numbers.rb) | Medium |

algorithms/ruby/1671-minimum-number-of-removals-to-make-mountain-array.rb

@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+
+# 1671. Minimum Number of Removals to Make Mountain Array
+# Hard
+# https://leetcode.com/problems/minimum-number-of-removals-to-make-mountain-array
+
+=begin
+You may recall that an array arr is a mountain array if and only if:
+* arr.length >= 3
+* There exists some index i (0-indexed) 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 an integer array nums​​​, return the minimum number of elements to remove to make nums​​​ a mountain array.
+
+Example 1:
+Input: nums = [1,3,1]
+Output: 0
+Explanation: The array itself is a mountain array so we do not need to remove any elements.
+
+Example 2:
+Input: nums = [2,1,1,5,6,2,3,1]
+Output: 3
+Explanation: One solution is to remove the elements at indices 0, 1, and 5, making the array nums = [1,5,6,3,1].
+
+Constraints:
+* 3 <= nums.length <= 1000
+* 1 <= nums[i] <= 109
+* It is guaranteed that you can make a mountain array out of nums.
+=end
+
+# @param {Integer[]} nums
+# @return {Integer}
+def minimum_mountain_removals(nums)
+  n = nums.size
+  inc = Array.new(n, 1)
+  dec = Array.new(n, 1)
+
+  (1...n).each do |i|
+    (0...i).each do |j|
+      inc[i] = [inc[i], inc[j] + 1].max if nums[i] > nums[j]
+    end
+  end
+
+  (n - 2).downto(0) do |i|
+    (i + 1...n).each do |j|
+      dec[i] = [dec[i], dec[j] + 1].max if nums[i] > nums[j]
+    end
+  end
+
+  max_mountain = 0
+  (1...n - 1).each do |i|
+    if inc[i] > 1 && dec[i] > 1
+      max_mountain = [max_mountain, inc[i] + dec[i] - 1].max
+    end
+  end
+
+  n - max_mountain
+end
+
+# ********************#
+#       TEST         #
+# ********************#
+
+require "test/unit"
+class Test_minimum_mountain_removals < Test::Unit::TestCase
+  def test_
+    assert_equal 0, minimum_mountain_removals([1, 3, 1])
+    assert_equal 3, minimum_mountain_removals([2, 1, 1, 5, 6, 2, 3, 1])
+  end
+end

algorithms/ruby/2684-maximum-number-of-moves-in-a-grid.rb

@@ -55,7 +55,7 @@ def max_moves(grid)
 require "test/unit"
 class Test_max_moves < Test::Unit::TestCase
   def test_
-    assert_equal 3, max_moves([[2,4,3,5],[5,4,9,3],[3,4,2,11],[10,9,13,15]])
-    assert_equal 0, max_moves([[3,2,4],[2,1,9],[1,1,7]])
+    assert_equal 3, max_moves([[2, 4, 3, 5], [5, 4, 9, 3], [3, 4, 2, 11], [10, 9, 13, 15]])
+    assert_equal 0, max_moves([[3, 2, 4], [2, 1, 9], [1, 1, 7]])
   end
 end