diff --git a/3sum/hyunjung-choi.kt b/3sum/hyunjung-choi.kt
new file mode 100644
index 000000000..1aff4131c
--- /dev/null
+++ b/3sum/hyunjung-choi.kt
@@ -0,0 +1,35 @@
+class Solution {
+    fun threeSum(nums: IntArray): List<List<Int>> {
+        if (nums.size < 3) return emptyList()
+
+        val result = mutableSetOf<List<Int>>()
+
+        nums.sort()
+
+        for (i in 0 until nums.size - 2) {
+
+            if (i > 0 && nums[i] == nums[i - 1]) continue
+
+            var left = i + 1
+            var right = nums.size - 1
+
+            while (left < right) {
+                val sum = nums[i] + nums[left] + nums[right]
+
+                when {
+                    sum == 0 -> {
+                        result.add(listOf(nums[i], nums[left], nums[right]))
+
+                        left++
+                        right--
+                    }
+
+                    sum < 0 -> left++
+                    else -> right--
+                }
+            }
+        }
+
+        return result.toList()
+    }
+}
diff --git a/climbing-stairs/hyunjung-choi.kt b/climbing-stairs/hyunjung-choi.kt
new file mode 100644
index 000000000..cadcc083a
--- /dev/null
+++ b/climbing-stairs/hyunjung-choi.kt
@@ -0,0 +1,16 @@
+class Solution {
+    fun climbStairs(n: Int): Int {
+        if (n <= 2) return n
+
+        var prev2 = 1
+        var prev1 = 2
+
+        for (i in 3..n) {
+            val current = prev1 + prev2
+            prev2 = prev1
+            prev1 = current
+        }
+
+        return prev1
+    }
+}
diff --git a/product-of-array-except-self/hyunjung-choi.kt b/product-of-array-except-self/hyunjung-choi.kt
new file mode 100644
index 000000000..1ac22486c
--- /dev/null
+++ b/product-of-array-except-self/hyunjung-choi.kt
@@ -0,0 +1,16 @@
+class Solution {
+    fun productExceptSelf(nums: IntArray): IntArray =
+        IntArray(nums.size) { 1 }.apply {
+            var leftProduct = 1
+            for (i in 1 until nums.size) {
+                leftProduct *= nums[i - 1]
+                this[i] = leftProduct
+            }
+
+            var rightProduct = 1
+            for (i in nums.size - 2 downTo 0) {
+                rightProduct *= nums[i + 1]
+                this[i] *= rightProduct
+            }
+        }
+}
diff --git a/valid-anagram/hyunjung-choi.kt b/valid-anagram/hyunjung-choi.kt
new file mode 100644
index 000000000..4357915c6
--- /dev/null
+++ b/valid-anagram/hyunjung-choi.kt
@@ -0,0 +1,19 @@
+class Solution {
+    fun isAnagram(s: String, t: String): Boolean {
+        if (s.length != t.length) return false
+
+        val charCount = mutableMapOf<Char, Int>()
+
+        s.forEach { ch ->
+            charCount.put(ch, charCount.getOrDefault(ch, 0) + 1)
+        }
+
+        t.forEach { ch ->
+            val count = charCount.getOrDefault(ch, 0)
+            if (count == 0) return false
+            charCount[ch] = count - 1
+        }
+
+        return true
+    }
+}
diff --git a/validate-binary-search-tree/hyunjung-choi.kt b/validate-binary-search-tree/hyunjung-choi.kt
new file mode 100644
index 000000000..0bce34a62
--- /dev/null
+++ b/validate-binary-search-tree/hyunjung-choi.kt
@@ -0,0 +1,30 @@
+/**
+ * Example:
+ * var ti = TreeNode(5)
+ * var v = ti.`val`
+ * Definition for a binary tree node.
+ * class TreeNode(var `val`: Int) {
+ *     var left: TreeNode? = null
+ *     var right: TreeNode? = null
+ * }
+ */
+class Solution {
+    fun isValidBST(root: TreeNode?): Boolean {
+        val values = mutableListOf<Int>()
+        inorderTraversal(root, values)
+
+        for (i in 1 until values.size) {
+            if (values[i] <= values[i - 1]) return false
+        }
+
+        return true
+    }
+
+    private fun inorderTraversal(root: TreeNode?, values: MutableList<Int>) {
+        if (root == null) return
+
+        inorderTraversal(root.left, values)
+        values.add(root.`val`)
+        inorderTraversal(root.right, values)
+    }
+}