316. Remove Duplicate Letters

Author: remy727

README.md

@@ -170,6 +170,7 @@
 | 295 | Find Median from Data Stream | [Ruby](./algorithms/ruby/0295-find-median-from-data-stream.rb) | Hard |
 | 299 | Bulls and Cows | [Ruby](./algorithms/ruby/0299-bulls-and-cows.rb) [Python3](./algorithms/python3/0299-bulls-and-cows.py) | Medium |
 | 300 | Longest Increasing Subsequence | [Ruby](./algorithms/ruby/0300-longest-increasing-subsequence.rb) | Medium |
+| 316 | Remove Duplicate Letters | [Ruby](./algorithms/ruby/0316-remove-duplicate-letters.rb) | Medium |
 | 319 | Bulb Switcher | [Ruby](./algorithms/ruby/0319-bulb-switcher.rb) | Medium |
 | 322 | Coin Change | [Ruby](./algorithms/ruby/0322-coin-change.rb) | Medium |
 | 328 | Odd Even Linked List | [Ruby](./algorithms/ruby/0328-odd-even-linked-list.rb) | Medium |

algorithms/ruby/0316-remove-duplicate-letters.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+# 316. Remove Duplicate Letters
+# Medium
+# https://leetcode.com/problems/remove-duplicate-letters
+
+=begin
+Given a string s, remove duplicate letters so that every letter appears once and only once. You must make sure your result is
+the smallest in lexicographical order among all possible results.
+
+Example 1:
+Input: s = "bcabc"
+Output: "abc"
+
+Example 2:
+Input: s = "cbacdcbc"
+Output: "acdb"
+
+Constraints:
+1 <= s.length <= 104
+s consists of lowercase English letters.
+=end
+
+# @param {String} s
+# @return {String}
+def remove_duplicate_letters(s)
+  count_hash = Hash.new { |h, k| h[k] = 0 }
+  stack = []
+  selected_set = Set.new
+
+  s.each_char do |char|
+    count_hash[char] += 1
+  end
+
+  s.each_char do |char|
+    count_hash[char] -= 1
+    unless selected_set.include?(char)
+      while !stack.empty? && count_hash[stack.last] > 0 && stack.last > char
+        selected_set.delete(stack.pop)
+      end
+
+      stack << char
+      selected_set << char
+    end
+  end
+
+  stack.join
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_length_of_longest_substring < Test::Unit::TestCase
+  def test_
+    assert_equal "abc", remove_duplicate_letters("bcabc")
+    assert_equal "acdb", remove_duplicate_letters("cbacdcbc")
+  end
+end