1964. Find the Longest Valid Obstacle Course at Each Position

Author: remy727

Diff for: README.md

@@ -182,6 +182,7 @@
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 | 1857 | Largest Color Value in a Directed Graph | [Ruby](./algorithms/ruby/1857-largest-color-value-in-a-directed-graph.rb) | Hard |
+| 1964 | Find the Longest Valid Obstacle Course at Each Position | [Ruby](./algorithms/ruby/1964-find-the-longest-valid-obstacle-course-at-each-position.rb) | Hard |
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Diff for: algorithms/ruby/1964-find-the-longest-valid-obstacle-course-at-each-position.rb

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+# frozen_string_literal: true
+
+# 1964. Find the Longest Valid Obstacle Course at Each Position
+# https://leetcode.com/problems/find-the-longest-valid-obstacle-course-at-each-position
+# Hard
+
+=begin
+You want to build some obstacle courses. You are given a 0-indexed integer array obstacles of length n, where obstacles[i] describes the height of the ith obstacle.
+
+For every index i between 0 and n - 1 (inclusive), find the length of the longest obstacle course in obstacles such that:
+
+You choose any number of obstacles between 0 and i inclusive.
+You must include the ith obstacle in the course.
+You must put the chosen obstacles in the same order as they appear in obstacles.
+Every obstacle (except the first) is taller than or the same height as the obstacle immediately before it.
+Return an array ans of length n, where ans[i] is the length of the longest obstacle course for index i as described above.
+
+Example 1:
+Input: obstacles = [1,2,3,2]
+Output: [1,2,3,3]
+Explanation: The longest valid obstacle course at each position is:
+- i = 0: [1], [1] has length 1.
+- i = 1: [1,2], [1,2] has length 2.
+- i = 2: [1,2,3], [1,2,3] has length 3.
+- i = 3: [1,2,3,2], [1,2,2] has length 3.
+
+Example 2:
+Input: obstacles = [2,2,1]
+Output: [1,2,1]
+Explanation: The longest valid obstacle course at each position is:
+- i = 0: [2], [2] has length 1.
+- i = 1: [2,2], [2,2] has length 2.
+- i = 2: [2,2,1], [1] has length 1.
+
+Example 3:
+Input: obstacles = [3,1,5,6,4,2]
+Output: [1,1,2,3,2,2]
+Explanation: The longest valid obstacle course at each position is:
+- i = 0: [3], [3] has length 1.
+- i = 1: [3,1], [1] has length 1.
+- i = 2: [3,1,5], [3,5] has length 2. [1,5] is also valid.
+- i = 3: [3,1,5,6], [3,5,6] has length 3. [1,5,6] is also valid.
+- i = 4: [3,1,5,6,4], [3,4] has length 2. [1,4] is also valid.
+- i = 5: [3,1,5,6,4,2], [1,2] has length 2.
+
+Constraints:
+n == obstacles.length
+1 <= n <= 105
+1 <= obstacles[i] <= 107
+=end
+
+# @param {Integer[]} obstacles
+# @return {Integer[]}
+def longest_obstacle_course_at_each_position(obstacles)
+  obstacle_path = []
+  ans = []
+  for i in 0..obstacles.length - 1
+    if obstacle_path.empty? || (obstacles[i] >= obstacle_path.last)
+      obstacle_path << obstacles[i]
+      ans[i] = obstacle_path.count
+    else
+      idx = obstacle_path.bsearch_index { |obstacle| obstacle > obstacles[i] }
+      obstacle_path[idx] = obstacles[i]
+      ans[i] = idx + 1
+    end
+  end
+
+  ans
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_longest_obstacle_course_at_each_position < Test::Unit::TestCase
+  def test_
+    assert_equal [1, 2, 3, 3], longest_obstacle_course_at_each_position([1, 2, 3, 2])
+    assert_equal [1, 2, 1], longest_obstacle_course_at_each_position([2, 2, 1])
+    assert_equal [1, 1, 2, 3, 2, 2], longest_obstacle_course_at_each_position([3, 1, 5, 6, 4, 2])
+  end
+end