diff --git a/study-plan/data-structure/day-1/contains-duplicate.rb b/study-plan/data-structure/day-01/contains-duplicate.rb
similarity index 100%
rename from study-plan/data-structure/day-1/contains-duplicate.rb
rename to study-plan/data-structure/day-01/contains-duplicate.rb
diff --git a/study-plan/data-structure/day-1/contains-duplicate.swift b/study-plan/data-structure/day-01/contains-duplicate.swift
similarity index 100%
rename from study-plan/data-structure/day-1/contains-duplicate.swift
rename to study-plan/data-structure/day-01/contains-duplicate.swift
diff --git a/study-plan/data-structure/day-1/maximum-subarray.rb b/study-plan/data-structure/day-01/maximum-subarray.rb
similarity index 100%
rename from study-plan/data-structure/day-1/maximum-subarray.rb
rename to study-plan/data-structure/day-01/maximum-subarray.rb
diff --git a/study-plan/data-structure/day-02/merge-sorted-array.rb b/study-plan/data-structure/day-02/merge-sorted-array.rb
new file mode 100644
index 0000000..5ab150e
--- /dev/null
+++ b/study-plan/data-structure/day-02/merge-sorted-array.rb
@@ -0,0 +1,59 @@
+# @param {Integer[]} nums1
+# @param {Integer} m
+# @param {Integer[]} nums2
+# @param {Integer} n
+# @return {Void} Do not return anything, modify nums1 in-place instead.
+def merge(nums1, m, nums2, n)
+    # shift numbers to right
+    i, k = m - 1, nums1.size - 1
+    while i >= 0 do
+        nums1[k] = nums1[i]
+        i -= 1
+        k -= 1
+    end
+
+    # merge them
+    i, j, k = nums1.size - m, 0, 0
+    while i < nums1.size && j < n do
+        if nums1[i] < nums2[j] then
+            nums1[k] = nums1[i]
+            i += 1
+        else
+            nums1[k] = nums2[j]
+            j += 1
+        end
+        k += 1
+    end
+
+    while j < n do
+        nums1[k] = nums2[j]
+        j += 1
+        k += 1
+    end
+end
+
+RSpec.describe "merge" do
+    it "nums1 = [1,2,3,0,0,0], m = 3, nums2 = [2,5,6], n = 3 ~> [1,2,2,3,5,6]" do
+        nums1, m = [1,2,3,0,0,0], 3
+        nums2, n = [2,5,6], 3
+
+        merge(nums1, m, nums2, n)
+        expect(nums1).to eq([1,2,2,3,5,6])
+    end
+
+    it "nums1 = [0], m = 0, nums2 = [1], n = 1 ~> [1]" do
+        nums1, m = [1], 1
+        nums2, n = [], 0
+
+        merge(nums1, m, nums2, n)
+        expect(nums1).to eq([1])
+    end
+
+    it "[3, 3] for 6 is [0, 1]" do
+        nums1, m = [0], 0
+        nums2, n = [1], 1
+
+        merge(nums1, m, nums2, n)
+        expect(nums1).to eq([1])
+    end
+end
diff --git a/study-plan/data-structure/day-02/two-sum.rb b/study-plan/data-structure/day-02/two-sum.rb
new file mode 100644
index 0000000..0ea7604
--- /dev/null
+++ b/study-plan/data-structure/day-02/two-sum.rb
@@ -0,0 +1,30 @@
+# @param {Integer[]} nums
+# @param {Integer} target
+# @return {Integer[]}
+def two_sum(nums, target)
+    encountered = Hash.new
+
+    nums.each_index { |i|
+        complement = encountered.assoc(target - nums[i])
+
+        if complement != nil then
+            return [complement[1], i]
+        else
+            encountered[nums[i]] = i
+        end
+    }
+end
+
+RSpec.describe "two_sum from" do
+    it "[2, 7, 11, 15] for 9 is [0, 1]" do
+        expect(two_sum([2, 7, 11, 15], 9)).to eq([0, 1])
+    end
+
+    it "[3, 2, 4] for 6 is [1, 2]" do
+        expect(two_sum([3, 2, 4], 6)).to eq([1, 2])
+    end
+
+    it "[3, 3] for 6 is [0, 1]" do
+        expect(two_sum([3, 3], 6)).to eq([0, 1])
+    end
+end