diff --git a/doitchuu/BestTimeToBuyAndSellStock.js b/doitchuu/BestTimeToBuyAndSellStock.js
new file mode 100644
index 0000000..7d78bda
--- /dev/null
+++ b/doitchuu/BestTimeToBuyAndSellStock.js
@@ -0,0 +1,20 @@
+/**
+ * @param {number[]} prices
+ * @return {number}
+ */
+var maxProfit = function(prices) {
+  // MEMO: 시간 초과!
+  let maxPrice = 0;
+
+  for (let i = 0; i < prices.length - 1; i++) {
+      for (let j = i + 1; j < prices.length; j++) {
+          const price = prices[i] - prices[j];
+
+          if (price < 0) {
+              maxPrice = Math.max(maxPrice, Math.abs(price));
+          }
+      }
+  }
+
+  return maxPrice;
+};
diff --git a/doitchuu/MergeTwoSortedLists.js b/doitchuu/MergeTwoSortedLists.js
new file mode 100644
index 0000000..fe0f383
--- /dev/null
+++ b/doitchuu/MergeTwoSortedLists.js
@@ -0,0 +1,15 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} list1
+ * @param {ListNode} list2
+ * @return {ListNode}
+ */
+var mergeTwoLists = function(list1, list2) {
+    // MEMO: 이 문제는 풀지 못했습니다.
+};
diff --git a/doitchuu/TwoSum.js b/doitchuu/TwoSum.js
index a260cdf..db22ab9 100644
--- a/doitchuu/TwoSum.js
+++ b/doitchuu/TwoSum.js
@@ -1,4 +1,16 @@
-// MEMO: 편하신 언어로 작성 부탁드립니다. 해당 주석은 풀이 입력 시 지워주세요.
-function twoSum(nums, target) {
-    
-}
+/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+var twoSum = function(nums, target) {
+  for (let i = 0; i < nums.length; i++) {
+      for (let j = i + 1; j < nums.length; j++) {
+          if (nums[i] + nums[j] === target) {
+              const output = [i, j];
+
+              return output;
+          }
+      }
+  }
+};
diff --git a/doitchuu/ValidParentheses.js b/doitchuu/ValidParentheses.js
new file mode 100644
index 0000000..a84fc14
--- /dev/null
+++ b/doitchuu/ValidParentheses.js
@@ -0,0 +1,35 @@
+
+/**
+ * @param {string} s
+ * @return {boolean}
+ */
+function isValid(s) {
+  if (s.length % 2 !== 0) {
+      return false;
+  }
+
+  let count = 0;
+  const stack = [];
+
+  for (let i = 0; i < s.length; i++) {
+      if (s[i] === "(" || s[i] === "{" || s[i] === "[") {
+          stack.push(s[i]);
+          count++;
+          continue;
+      }
+
+      if (stack[stack.length - 1] === "(" && s[i] === ")") {
+          stack.pop();
+      }
+
+      if (stack[stack.length - 1] === "{" && s[i] === "}") {
+          stack.pop();
+      }
+
+      if (stack[stack.length - 1] === "[" && s[i] === "]") {
+          stack.pop();
+      }
+  }
+
+  return stack.length > 0 && count !== (s.length / 2) ? false : true;
+};