diff --git a/study-plan/data-structure/day-1/contains-duplicate.rb b/study-plan/data-structure/day-1/contains-duplicate.rb
new file mode 100644
index 0000000..6a44b8e
--- /dev/null
+++ b/study-plan/data-structure/day-1/contains-duplicate.rb
@@ -0,0 +1,27 @@
+# @param {Integer[]} nums
+# @return {Boolean}
+def contains_duplicate(nums)
+    encountered = Set.new
+
+    nums.each { |x|
+        if encountered.add?(x) == nil then
+            return true
+        end
+    }
+
+    return false
+end
+
+RSpec.describe "contains_duplicate" do
+    it "nums = [1,2,3,1] contains" do
+        expect(contains_duplicate([1,2,3,1])).to be true
+    end
+
+    it "nums = [1,2,3,4] doesn't contain" do
+        expect(contains_duplicate([1,2,3,4])).to be false
+    end
+
+    it "nums = [1,1,1,3,3,4,3,2,4,2] contains" do
+        expect(contains_duplicate([1,1,1,3,3,4,3,2,4,2])).to be true
+    end
+end
diff --git a/problems/contains-duplicate.swift b/study-plan/data-structure/day-1/contains-duplicate.swift
similarity index 100%
rename from problems/contains-duplicate.swift
rename to study-plan/data-structure/day-1/contains-duplicate.swift
diff --git a/study-plan/data-structure/day-1/maximum-subarray.rb b/study-plan/data-structure/day-1/maximum-subarray.rb
new file mode 100644
index 0000000..99124db
--- /dev/null
+++ b/study-plan/data-structure/day-1/maximum-subarray.rb
@@ -0,0 +1,29 @@
+# @param {Integer[]} nums
+# @return {Integer}
+def max_sub_array(nums)
+    if nums.empty? then
+        return nil
+    end
+
+    sum, running_sum = nums.first, nums.first
+    nums.drop(1).each { |x|
+        running_sum = [running_sum + x, x].max
+        sum = [sum, running_sum].max
+    }
+
+    return sum
+end
+
+RSpec.describe "max_sub_array of " do
+    it "[-2,1,-3,4,-1,2,1,-5,4] is 6" do
+        expect(max_sub_array([-2,1,-3,4,-1,2,1,-5,4])).to eq(6)
+    end
+
+    it "[1] is 1" do
+        expect(max_sub_array([1])).to eq(1)
+    end
+
+    it "[5,4,-1,7,8] is 23" do
+        expect(max_sub_array([5,4,-1,7,8])).to eq(23)
+    end
+end