Merge pull request #94 from jsca-kwok/jk-find-all-duplicates-in-array

Find All Duplicates in an Array

data_structures/arrays/find_all_duplicates_in_an_array.rb

@@ -0,0 +1,130 @@
+# Find All Duplicates in an Array
+#
+# Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array),
+# some elements appear twice and others appear once.
+#
+# Find all the elements that appear twice in this array.
+#
+# Could you do it without extra space and in O(n) runtime?
+#
+# Example:
+# Input:
+# [4,3,2,7,8,2,3,1]
+#
+# Output:
+# [2,3]
+
+require 'benchmark'
+
+array = [4, 3, 2, 7, 8, 2, 3, 1]
+long_array = [4, 3, 2, 7, 8, 2, 3, 1] * 100
+
+#
+# Approach 1: Brute force
+#
+
+#
+# Complexity Analysis
+#
+# Time complexity: O(n^2) average case.
+#
+
+def find_duplicates(array)
+  current_num = array[0]
+  result_array = []
+
+  array.count.times do |i|
+    array.count.times do |j|
+      next if i == j || current_num != array[j]
+
+      result_array.push(current_num)
+    end
+
+    current_num = array[i + 1]
+  end
+
+  result_array.uniq
+end
+
+Benchmark.bmbm do |x|
+  x.report('execute algorithm 1') do
+    print(find_duplicates(array))
+    print(find_duplicates(long_array))
+  end
+end
+
+#
+# Approach 2: Sort and Compare Adjacent Elements
+#
+
+# Intuition
+
+# After sorting a list of elements, all elements of equivalent value get placed together.
+# Thus, when you sort an array, equivalent elements form contiguous blocks.
+
+#
+# Complexity Analysis
+#
+# Time complexity: O(n log n)
+#
+
+def find_duplicates_2(array)
+  sorted_array = array.sort
+  result_array = []
+
+  (1..sorted_array.count).each do |i|
+    next if sorted_array[i] != sorted_array[i - 1]
+
+    result_array.push(sorted_array[i])
+  end
+
+  result_array.uniq
+end
+
+Benchmark.bmbm do |x|
+  x.report('execute algorithm 2') do
+    print(find_duplicates(array))
+    print(find_duplicates(long_array))
+  end
+end
+
+#
+# Approach 3: Hash map
+#
+
+#
+# Complexity Analysis
+#
+# Time complexity: O(n) average case.
+#
+
+def find_duplicates_3(array)
+  result_hash = {}
+  result_array = []
+
+  # loop through array and build a hash with counters
+  # where the key is the array element and the counter is the value
+  # increase counter when duplicate is found
+  array.each do |num|
+    if result_hash[num].nil?
+      result_hash[num] = 1
+    else
+      result_hash[num] += 1
+    end
+  end
+
+  # loop through hash and look for values > 1
+  result_hash.each do |k, v|
+    result_array.push(k) if v > 1
+  end
+
+  # return keys
+  result_array
+end
+
+Benchmark.bmbm do |x|
+  x.report('execute algorithm 3') do
+    print(find_duplicates(array))
+    print(find_duplicates(long_array))
+  end
+end