2501. Longest Square Streak in an Array (#189)

Author: remy727

README.md

@@ -494,6 +494,7 @@
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+| 2501 | Longest Square Streak in an Array | [Ruby](./algorithms/ruby/2501-longest-square-streak-in-an-array.rb) | Medium |
 | 2542 | Maximum Subsequence Score | [Ruby](./algorithms/ruby/2542-maximum-subsequence-score.rb) | Medium |
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algorithms/ruby/2501-longest-square-streak-in-an-array.rb

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+# frozen_string_literal: true
+
+# 2501. Longest Square Streak in an Array
+# Medium
+# https://leetcode.com/problems/longest-square-streak-in-an-array
+
+=begin
+You are given an integer array nums. A subsequence of nums is called a square streak if:
+* The length of the subsequence is at least 2, and
+* after sorting the subsequence, each element (except the first element) is the square of the previous number.
+Return the length of the longest square streak in nums, or return -1 if there is no square streak.
+A subsequence is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.
+
+Example 1:
+Input: nums = [4,3,6,16,8,2]
+Output: 3
+Explanation: Choose the subsequence [4,16,2]. After sorting it, it becomes [2,4,16].
+- 4 = 2 * 2.
+- 16 = 4 * 4.
+Therefore, [4,16,2] is a square streak.
+It can be shown that every subsequence of length 4 is not a square streak.
+
+Example 2:
+Input: nums = [2,3,5,6,7]
+Output: -1
+Explanation: There is no square streak in nums so return -1.
+
+Constraints:
+* 2 <= nums.length <= 105
+* 2 <= nums[i] <= 105
+=end
+
+# @param {Integer[]} nums
+# @return {Integer}
+def longest_square_streak(nums)
+  nums = nums.uniq.sort
+  num_set = nums.to_set
+  max_length = 0
+
+  nums.each do |num|
+    length = 0
+    current = num
+
+    while num_set.include?(current)
+      length += 1
+      current = current**2
+    end
+
+    if length > 1
+      max_length = [max_length, length].max
+    end
+  end
+
+  max_length > 1 ? max_length : -1
+end
+
+# ********************#
+#       TEST         #
+# ********************#
+
+require "test/unit"
+class Test_longest_square_streak < Test::Unit::TestCase
+  def test_
+    assert_equal 3, longest_square_streak([4, 3, 6, 16, 8, 2])
+    assert_equal(-1, longest_square_streak([2, 3, 5, 6, 7]))
+  end
+end