From 9429ae92c1769f47c0d8465de18a576e55a68ff8 Mon Sep 17 00:00:00 2001
From: Jessica Kwok <jessicatckwok@gmail.com>
Date: Fri, 12 Mar 2021 18:51:41 -0800
Subject: [PATCH 1/3] Add remove elements challenge instructions

---
 DIRECTORY.md                              |  3 ++-
 data_structures/arrays/remove_elements.rb | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 data_structures/arrays/remove_elements.rb

diff --git a/DIRECTORY.md b/DIRECTORY.md
index 6045aa6..fd7a1dd 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -9,7 +9,8 @@
     * [Get Products Of All Other Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/get_products_of_all_other_elements.rb)
     * [Sort Squares Of An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/sort_squares_of_an_array.rb)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum.rb)
-    * [Two Sum Ii](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum_ii.rb)
+    * [Two Sum II](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum_ii.rb)
+    * [Remove Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/remove_elements.rb)
   * Binary Trees
     * [Inorder Traversal](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/binary_trees/inorder_traversal.rb)
     * [Invert](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/binary_trees/invert.rb)
diff --git a/data_structures/arrays/remove_elements.rb b/data_structures/arrays/remove_elements.rb
new file mode 100644
index 0000000..e407c7b
--- /dev/null
+++ b/data_structures/arrays/remove_elements.rb
@@ -0,0 +1,18 @@
+# Arrays - Remove Elements
+#
+# Given an array nums and a value val, remove all instances of that value in-place and return the new length.
+# Do not allocate extra space for another array, 
+# you must do this by modifying the input array in-place with O(1) extra memory.
+# The order of elements can be changed. It doesn't matter what you leave beyond the new length.
+#
+# Example
+#
+# Input: nums = [3,2,2,3], val = 3
+# Output: 2, nums = [2,2]
+#
+# Input: nums = [0,1,2,2,3,0,4,2], val = 2
+# Output: 5, nums = [0,1,4,0,3]
+
+#
+# Approach 1: Brute Force
+#
\ No newline at end of file

From ba03727fb6cef1ab4a06f994ffe549030f53f163 Mon Sep 17 00:00:00 2001
From: Jessica Kwok <jessicatckwok@gmail.com>
Date: Mon, 15 Mar 2021 09:52:22 -0700
Subject: [PATCH 2/3] Add solution using delete_if Ruby method

---
 data_structures/arrays/remove_elements.rb | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/data_structures/arrays/remove_elements.rb b/data_structures/arrays/remove_elements.rb
index e407c7b..84abca7 100644
--- a/data_structures/arrays/remove_elements.rb
+++ b/data_structures/arrays/remove_elements.rb
@@ -14,5 +14,16 @@
 # Output: 5, nums = [0,1,4,0,3]
 
 #
-# Approach 1: Brute Force
-#
\ No newline at end of file
+# Approach 1: Use `delete_if` Ruby method
+#
+# Time complexity: O(n)
+#
+def remove_elements(nums, val)
+  nums.delete_if{ |num| num == val }
+  nums.length
+end
+
+puts remove_elements([3,2,2,3], 3)
+# => 2
+puts remove_elements([0,1,2,2,3,0,4,2], 2)
+# => 5
\ No newline at end of file

From 1f95ad15df10818ed7bac6f66dc37e05c8a97f79 Mon Sep 17 00:00:00 2001
From: Jessica Kwok <jessicatckwok@gmail.com>
Date: Mon, 15 Mar 2021 10:08:31 -0700
Subject: [PATCH 3/3] Add solution using delete_at Ruby method

---
 data_structures/arrays/remove_elements.rb | 27 ++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/data_structures/arrays/remove_elements.rb b/data_structures/arrays/remove_elements.rb
index 84abca7..32260ac 100644
--- a/data_structures/arrays/remove_elements.rb
+++ b/data_structures/arrays/remove_elements.rb
@@ -26,4 +26,29 @@ end
 puts remove_elements([3,2,2,3], 3)
 # => 2
 puts remove_elements([0,1,2,2,3,0,4,2], 2)
-# => 5
\ No newline at end of file
+# => 5
+
+#
+# Approach 2: Use `delete_at`, `unshift`, and `shift` Ruby method
+#
+# Time complexity: O(n)
+#
+def remove_elements(nums, val)
+  result_length = nums.length
+  shift_length = 0
+  nums.each_with_index do |num, i|
+    if num == val 
+      nums.delete_at(i)
+      nums.unshift('removed')
+      result_length -=1
+      shift_length += 1
+    end
+  end
+  nums.shift(shift_length)
+  result_length
+end
+
+puts remove_elements([3,2,2,3], 3)
+# => 2
+puts remove_elements([0,1,2,2,3,0,4,2], 2)
+# => 5