389. Find the Difference

Author: remy727

README.md

@@ -186,6 +186,7 @@
 | 382 | Linked List Random Node | [Ruby](./algorithms/ruby/0382-linked-list-random-node.rb) | Medium |
 | 383 | Ransom Note | [Ruby](./algorithms/ruby/0383-ransom-note.rb) | Easy |
 | 387 | First Unique Character in a String | [Ruby](./algorithms/ruby/0387-first-unique-character-in-a-string.rb) | Easy |
+| 389 | Find the Difference | [Ruby](./algorithms/ruby/0389-find-the-difference.rb) | Easy |
 | 392 | Is Subsequence | [Ruby](./algorithms/ruby/0392-is-subsequence.rb) | Easy |
 | 394 | Decode String | [Ruby](./algorithms/ruby/0394-decode-string.rb) | Medium |
 | 399 | Evaluate Division | [Ruby](./algorithms/ruby/0399-evaluate-division.rb) | Medium |

algorithms/ruby/0389-find-the-difference.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+# 389. Find the Difference
+# Easy
+# https://leetcode.com/problems/find-the-difference
+
+=begin
+You are given two strings s and t.
+
+String t is generated by random shuffling string s and then add one more letter at a random position.
+Return the letter that was added to t.
+
+Example 1:
+Input: s = "abcd", t = "abcde"
+Output: "e"
+Explanation: 'e' is the letter that was added.
+
+Example 2:
+Input: s = "", t = "y"
+Output: "y"
+
+Constraints:
+0 <= s.length <= 1000
+t.length == s.length + 1
+s and t consist of lowercase English letters.
+=end
+
+# @param {String} s
+# @param {String} t
+# @return {Character}
+def find_the_difference(s, t)
+  (s + t).bytes.reduce(:^).chr
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_find_the_difference < Test::Unit::TestCase
+  def test_
+    assert_equal "e", find_the_difference("abcd", "abcde")
+    assert_equal "y", find_the_difference("", "y")
+  end
+end