1921. Eliminate Maximum Number of Monsters

README.md

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 | 1845 | Seat Reservation Manager | [Ruby](./algorithms/ruby/1845-seat-reservation-manager.rb) | Medium |
 | 1857 | Largest Color Value in a Directed Graph | [Ruby](./algorithms/ruby/1857-largest-color-value-in-a-directed-graph.rb) | Hard |
 | 1870 | Minimum Speed to Arrive on Time | [Ruby](./algorithms/ruby/1870-minimum-speed-to-arrive-on-time.rb) | Medium |
+| 1921 | Eliminate Maximum Number of Monsters | [Ruby](./algorithms/ruby/1921-eliminate-maximum-number-of-monsters.rb) | Medium |
 | 1926 | Nearest Exit from Entrance in Maze | [Ruby](./algorithms/ruby/1926-nearest-exit-from-entrance-in-maze.rb) | Medium |
 | 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 |
 | 1970 | Last Day Where You Can Still Cross | [Ruby](./algorithms/ruby/1970-last-day-where-you-can-still-cross.rb) | Hard |

algorithms/ruby/1921-eliminate-maximum-number-of-monsters.rb

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+# frozen_string_literal: true
+
+# 1921. Eliminate Maximum Number of Monsters
+# Medium
+# https://leetcode.com/problems/eliminate-maximum-number-of-monsters
+
+=begin
+You are playing a video game where you are defending your city from a group of n monsters. You are given a 0-indexed integer array dist of size n, where dist[i] is the initial distance in kilometers of the ith monster from the city.
+The monsters walk toward the city at a constant speed. The speed of each monster is given to you in an integer array speed of size n, where speed[i] is the speed of the ith monster in kilometers per minute.
+You have a weapon that, once fully charged, can eliminate a single monster. However, the weapon takes one minute to charge. The weapon is fully charged at the very start.
+You lose when any monster reaches your city. If a monster reaches the city at the exact moment the weapon is fully charged, it counts as a loss, and the game ends before you can use your weapon.
+Return the maximum number of monsters that you can eliminate before you lose, or n if you can eliminate all the monsters before they reach the city.
+
+Example 1:
+Input: dist = [1,3,4], speed = [1,1,1]
+Output: 3
+Explanation:
+In the beginning, the distances of the monsters are [1,3,4]. You eliminate the first monster.
+After a minute, the distances of the monsters are [X,2,3]. You eliminate the second monster.
+After a minute, the distances of the monsters are [X,X,2]. You eliminate the thrid monster.
+All 3 monsters can be eliminated.
+
+Example 2:
+Input: dist = [1,1,2,3], speed = [1,1,1,1]
+Output: 1
+Explanation:
+In the beginning, the distances of the monsters are [1,1,2,3]. You eliminate the first monster.
+After a minute, the distances of the monsters are [X,0,1,2], so you lose.
+You can only eliminate 1 monster.
+
+Example 3:
+Input: dist = [3,2,4], speed = [5,3,2]
+Output: 1
+Explanation:
+In the beginning, the distances of the monsters are [3,2,4]. You eliminate the first monster.
+After a minute, the distances of the monsters are [X,0,2], so you lose.
+You can only eliminate 1 monster.
+
+
+Constraints:
+n == dist.length == speed.length
+1 <= n <= 105
+1 <= dist[i], speed[i] <= 105
+=end
+
+# @param {Integer[]} dist
+# @param {Integer[]} speed
+# @return {Integer}
+def eliminate_maximum(dist, speed)
+ return 0 if dist.empty? || speed.empty?
+
+ time = dist.map.with_index { |d, i| d.to_f / speed[i] }
+
+ time.sort!
+
+ count = 0
+ time.each_with_index do |t, i|
+ break if t <= i
+
+ count += 1
+ end
+
+ count
+end
+
+# **************** #
+# TEST #
+# **************** #
+
+require "test/unit"
+class Test_eliminate_maximum < Test::Unit::TestCase
+ def test_
+ assert_equal 3, eliminate_maximum([1, 3, 4], [1, 1, 1])
+ assert_equal 1, eliminate_maximum([1, 1, 2, 3], [1, 1, 1, 1])
+ assert_equal 1, eliminate_maximum([3, 2, 4], [5, 3, 2])
+ end
+end