From a6ae1c25b5053fd69135f56dca7dc005f812bbec Mon Sep 17 00:00:00 2001
From: Jessica Kwok <jessicatckwok@gmail.com>
Date: Thu, 11 Mar 2021 15:53:01 -0800
Subject: [PATCH] Add solution using a hash

---
 data_structures/arrays/two_sum.rb | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/data_structures/arrays/two_sum.rb b/data_structures/arrays/two_sum.rb
index 6fec6fc..adc5eb4 100644
--- a/data_structures/arrays/two_sum.rb
+++ b/data_structures/arrays/two_sum.rb
@@ -24,7 +24,7 @@
 # @return {Integer[]}
 
 # 
-# Approach 1: Brute Force
+# Approach 1: Brute Force with Addition
 #
 # Time Complexity: O(N^2), where N is the length of the array
 #
@@ -51,7 +51,7 @@ print two_sum([3, 3], 6)
 # => [0,1]
 
 #
-# Approach 2: Define and seek
+# Approach 2: Brute Force with Difference
 #
 # Time Complexity: O(N^2), where N is the length of the array
 #
@@ -66,6 +66,31 @@ def two_sum(nums, target)
   end
 end
 
+print two_sum([2, 7, 11, 15], 9) 
+# => [0,1]
+print two_sum([3, 2, 4], 6)
+# => [1,2]
+print two_sum([3, 3], 6)
+# => [0,1]
+
+#
+# Approach 3: Using a Hash
+#
+# Time Complexity: O(N), where N is the length of the array
+#
+def two_sum(nums, target)
+  hash = {}
+  # create a hash to store values and their indices
+  nums.each_with_index do |num, i|
+    hash[num] = i
+  end
+  # iterate over nums array to find the target (difference between sum target and num)
+  nums.each_with_index do |num, i|
+    difference_target = target - num
+    return [i, hash[difference_target]] if hash[difference_target] && hash[difference_target] != i
+  end
+end
+
 print two_sum([2, 7, 11, 15], 9) 
 # => [0,1]
 print two_sum([3, 2, 4], 6)