diff --git a/contains-duplicate/sunjae95.js b/contains-duplicate/sunjae95.js
new file mode 100644
index 000000000..ecea62667
--- /dev/null
+++ b/contains-duplicate/sunjae95.js
@@ -0,0 +1,22 @@
+/**
+ * @description
+ * time complexity: O(n)
+ * space complexity: O(n)
+ * approach/strategy:
+ * 1. brute force + hash table
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {boolean}
+ */
+var containsDuplicate = function (nums) {
+  const hashTable = new Set();
+
+  for (const num of nums) {
+    if (hashTable.has(num)) return true;
+    hashTable.add(num);
+  }
+
+  return false;
+};
diff --git a/kth-smallest-element-in-a-bst/sunjae95.js b/kth-smallest-element-in-a-bst/sunjae95.js
new file mode 100644
index 000000000..cbf4996f7
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/sunjae95.js
@@ -0,0 +1,33 @@
+/**
+ * @description
+ * time complexity: O(N)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. BFS, DFS
+ * 2. Brute force
+ *
+ * strategy:
+ * inOrder search
+ *
+ * reason:
+ * tree features
+ */
+var kthSmallest = function (root, k) {
+  let answer = 0;
+
+  inOrder(root, (value) => {
+    k -= 1;
+    if (k > 0) return false;
+    if (k === 0) answer = value;
+    return true;
+  });
+
+  return answer;
+};
+
+function inOrder(tree, isEnd) {
+  if (tree.left) inOrder(tree.left, isEnd);
+  if (isEnd(tree.val)) return;
+  if (tree.right) inOrder(tree.right, isEnd);
+}
diff --git a/number-of-1-bits/sunjae95.js b/number-of-1-bits/sunjae95.js
new file mode 100644
index 000000000..3d6ef7f2b
--- /dev/null
+++ b/number-of-1-bits/sunjae95.js
@@ -0,0 +1,22 @@
+/**
+ * @description
+ * time complexity: O(logN)
+ * space complexity: O(1)
+ * approach/strategy:
+ * 1. decimal to binary
+ */
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var hammingWeight = function (n) {
+  let answer = 0;
+
+  while (n > 0) {
+    answer += n % 2;
+    n = Math.floor(n / 2);
+  }
+
+  return answer;
+};
diff --git a/palindromic-substrings/sunjae95.js b/palindromic-substrings/sunjae95.js
new file mode 100644
index 000000000..1e3598be0
--- /dev/null
+++ b/palindromic-substrings/sunjae95.js
@@ -0,0 +1,90 @@
+/**
+ * @description
+ * time complexity: O(N^3)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. stack, permutation
+ * 2. Brute force
+ *
+ * strategy:
+ * Brute force, calculate
+ *
+ * reason:
+ * intuitive way
+ *
+ * @param {string} s
+ * @return {number}
+ */
+var countSubstrings = function (s) {
+  let answer = 0;
+  const len = s.length;
+
+  for (let i = 0; i < len; i++) {
+    for (let j = i + 1; j < len + 1; j++) {
+      const subStr = s.slice(i, j);
+      if (isPalindrome(subStr)) answer++;
+    }
+  }
+
+  return answer;
+};
+
+function isPalindrome(str) {
+  const len = str.length;
+  const middleIndex = Math.floor(len / 2);
+
+  for (let i = 0; i < middleIndex; i++) {
+    if (str[i] !== str[len - 1 - i]) return false;
+  }
+
+  return true;
+}
+
+/**
+ * @description
+ * time complexity: O(N^2)
+ * space complexity: O(N^2)
+ *
+ * brainstorming:
+ * 1. https://sbslc.tistory.com/56
+ *
+ * strategy:
+ * dynamic programming
+ *
+ * reason:
+ * to challenge dp
+ *
+ * @param {string} s
+ * @return {number}
+ */
+var countSubstrings = function (s) {
+  const answer = [];
+  const MAX_LENGTH = s.length;
+  const dp = Array.from({ length: MAX_LENGTH }, (_, i) =>
+    Array.from({ length: MAX_LENGTH }, (_, j) => {
+      if (i === j) answer.push(s[i]);
+      return i === j;
+    })
+  );
+  // Check next step ex) aa, bb, cc
+  for (let i = 0; i < MAX_LENGTH - 1; i++) {
+    const nextIndex = i + 1;
+    if (s[i] === s[nextIndex]) {
+      dp[i][nextIndex] = true;
+      answer.push(s.slice(i, nextIndex + 1));
+    }
+  }
+  // Check against values calculated in the previous step
+  for (let len = 2; len <= MAX_LENGTH; len++) {
+    for (let i = 0; i < MAX_LENGTH - len; i++) {
+      const lastIndex = len + i;
+      if (s[i] === s[lastIndex] && dp[i + 1][lastIndex - 1]) {
+        dp[i][lastIndex] = true;
+        answer.push(s.slice(i, lastIndex + 1));
+      }
+    }
+  }
+
+  return answer.length;
+};
diff --git a/top-k-frequent-elements/sunjae95.js b/top-k-frequent-elements/sunjae95.js
new file mode 100644
index 000000000..2d449ff44
--- /dev/null
+++ b/top-k-frequent-elements/sunjae95.js
@@ -0,0 +1,29 @@
+/**
+ * @description
+ * time complexity: O(N logN)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. javascript sort method
+ * 2. priority queue
+ *
+ * strategy:
+ * javascript sort method
+ *
+ * reason:
+ * javascript sort method is easier to implement.
+ */
+
+var topKFrequent = function (nums, k) {
+  const answer = [];
+  const hashTable = new Map();
+
+  nums.forEach((num) => hashTable.set(num, (hashTable.get(num) ?? 0) + 1));
+
+  hashTable.forEach((count, number) => answer.push({ number, count }));
+
+  return answer
+    .sort((a, b) => b.count - a.count)
+    .slice(0, k)
+    .map(({ number }) => number);
+};