K Closest Points to Origin

Author: rayning0

Gemfile

@@ -4,3 +4,4 @@ gem 'rspec'
 gem 'ruby-debug-ide'
 gem 'debase'
 gem 'pry-byebug'
+gem 'algorithms'

Gemfile.lock

@@ -1,6 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    algorithms (0.6.1)
     byebug (11.1.1)
     coderay (1.1.2)
     debase (0.2.4.1)
@@ -35,6 +36,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  algorithms
   debase
   pry-byebug
   rspec

leetcode/k_closest_points_spec.rb

@@ -0,0 +1,47 @@
+# https://leetcode.com/problems/k-closest-points-to-origin/
+# https://www.educative.io/courses/grokking-the-coding-interview/3YxNVYwNR5p
+# https://medium.com/basecs/learning-to-love-heaps-cef2b273a238
+# https://www.hackerearth.com/practice/data-structures/trees/heapspriority-queues/tutorial/
+# https://www.youtube.com/watch?v=t0Cq6tVNRBA
+# Runtime: 220 ms, faster than 100.00% of Ruby online submissions for K Closest Points to Origin.
+# Memory Usage: 12.1 MB, less than 100.00% of Ruby online submissions for K Closest Points to Origin.
+
+# O(n log n)
+def k_closest(points, k)
+  points.sort_by{|x, y| x*x + y*y}.first(k)
+end
+
+# http://kanwei.github.io/algorithms/classes/Containers/MinHeap.html
+require 'algorithms'
+include Containers
+
+# O(n log n + k log k) = O(n log n)
+def k_closest2(points, k)
+  heap = MinHeap.new
+  points.each do |x, y|
+    d = x*x + y*y
+    heap.push(d, [x, y])
+  end
+  ans = []
+  k.times do
+    ans << heap.pop
+  end
+  ans
+end
+
+describe '#k_closest' do
+  it 'finds k closest points to origin (0,0)' do
+    expect(k_closest([[0,1],[1,0]], 2)).to eq [[0,1], [1,0]]
+    expect(k_closest([[1,3],[-2,2]], 1)).to eq [[-2,2]]
+
+    now = Time.now
+    expect(k_closest([[3,3],[5,-1],[-2,4]], 2)).to eq [[3,3],[-2,4]]
+    puts Time.now - now
+    # 1.2e-05 secs
+
+    now = Time.now
+    expect(k_closest2([[3,3],[5,-1],[-2,4]], 2)).to eq [[3,3],[-2,4]]
+    puts Time.now - now
+    # 8.0e-05 secs (heap is slower)
+  end
+end