diff --git a/3sum/hoyeongkwak.java b/3sum/hoyeongkwak.java
new file mode 100644
index 000000000..a818933d0
--- /dev/null
+++ b/3sum/hoyeongkwak.java
@@ -0,0 +1,32 @@
+/*
+Time Complexity : O(n^2)
+Space Complexity : O(1)
+*/
+class Solution {
+    public List<List<Integer>> threeSum(int[] nums) {
+        List<List<Integer>> result = new ArrayList<>();
+        Arrays.sort(nums);
+
+        for (int i = 0; i < nums.length - 2; i++) {
+            if (i > 0 && nums[i] == nums[i - 1]) continue;
+            int low = i + 1;
+            int high = nums.length - 1;
+            while (low < high) {
+                int threeSum = nums[i] + nums[low] + nums[high];
+                if (threeSum < 0) {
+                    low = low + 1;
+                } else if (threeSum > 0) {
+                    high = high - 1;
+                } else {
+                    result.add(Arrays.asList(nums[i], nums[low], nums[high]));
+                    while (low < high && nums[low] == nums[low + 1]) low++;
+                    while (low < high && nums[high] == nums[high - 1]) high--;
+                    low = low + 1;
+                    high = high - 1;
+                }
+             }
+        }
+
+        return result;
+    }
+}
diff --git a/climbing-stairs/hoyeongkwak.java b/climbing-stairs/hoyeongkwak.java
new file mode 100644
index 000000000..940956613
--- /dev/null
+++ b/climbing-stairs/hoyeongkwak.java
@@ -0,0 +1,18 @@
+/*
+Time Complexity : O(n)
+Space Complexity : O(1)
+*/
+class Solution {
+    public int climbStairs(int n) {
+        if (n < 3) return n;
+        int prev = 1;
+        int curr = 2;
+
+        for(int i = 0; i < n - 2; i++) {
+            int temp = prev;
+            prev = curr;
+            curr = temp + curr;
+        }
+        return curr;
+    }
+}
diff --git a/product-of-array-except-self/hoyeongkwak.java b/product-of-array-except-self/hoyeongkwak.java
new file mode 100644
index 000000000..d4c189834
--- /dev/null
+++ b/product-of-array-except-self/hoyeongkwak.java
@@ -0,0 +1,23 @@
+/*
+Time Complexity : O(n)
+Space Complexity : O(n)
+*/
+class Solution {
+    public int[] productExceptSelf(int[] nums) {
+        int[] results = new int[nums.length];
+        Arrays.fill(results, 1);
+        int before = 1;
+        int after = 1;
+
+        for (int i = 0; i < nums.length - 1; i++) {
+            before = before * nums[i];
+            results[i + 1] = results[i + 1] * before;
+        }
+
+        for (int i = nums.length - 1; i > 0; i--) {
+            after = after * nums[i];
+            results[i - 1] = results[i - 1] * after;
+        }
+        return results;
+    }
+}
diff --git a/valid-anagram/hoyeongkwak.java b/valid-anagram/hoyeongkwak.java
new file mode 100644
index 000000000..5e8f7c1ca
--- /dev/null
+++ b/valid-anagram/hoyeongkwak.java
@@ -0,0 +1,28 @@
+/*
+- 애너그램 또는 어구정철 : 문장의 순서를 바꾸어 다른 단어로 만드는 것
+- 같은 단어가 맞는지 검증
+- s와 t의 각 알파벳별로 한쪽은 증가, 한쪽은 감소하면서 갯수 세기
+- 갯수가 0이 아닌 경우엔 서로 다른 단어이기에 false
+*/
+/*
+Time Complexity : O(n)
+Space Complexity : O(1)
+*/
+class Solution {
+    public boolean isAnagram(String s, String t) {
+        if (s.length() != t.length())
+            return false;
+
+        int[] count = new int[26];
+        for (int i = 0; i < s.length(); i++) {
+            count[s.charAt(i) - 'a']++;
+            count[t.charAt(i) - 'a']--;
+        }
+
+        for (int c : count) {
+            if (c != 0)
+                return false;
+        }
+        return true;
+    }
+}
diff --git a/validate-binary-search-tree/hoyeongkwak.java b/validate-binary-search-tree/hoyeongkwak.java
new file mode 100644
index 000000000..f20159584
--- /dev/null
+++ b/validate-binary-search-tree/hoyeongkwak.java
@@ -0,0 +1,29 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+ /*
+Time Complexity : O(n)
+Space Complexity : O(n)
+*/
+class Solution {
+    TreeNode prev;
+    public boolean isValidBST(TreeNode root) {
+        if (root == null) return true;
+        if (!isValidBST(root.left)) return false;
+        if (prev != null && prev.val >= root.val) return false;
+        prev = root;
+        return isValidBST(root.right);
+    }
+}