diff --git a/climbing-stairs/gitsunmin.ts b/climbing-stairs/gitsunmin.ts
new file mode 100644
index 000000000..e87319fa4
--- /dev/null
+++ b/climbing-stairs/gitsunmin.ts
@@ -0,0 +1,17 @@
+/**
+ * https://leetcode.com/problems/climbing-stairs
+ * time complexity : O(n)
+ * space complexity : O(1)
+ */
+
+export const upStairs = (n: number): number => {
+    let [l, r] = [1, 2];
+    for (let i = 3; i <= n; i++) [l, r] = [r, l + r];
+
+    return r;
+};
+
+export function climbStairs(n: number): number {
+    if (n <= 2) return n;
+    return upStairs(n);
+};
diff --git a/product-of-array-except-self/gitsunmin.ts b/product-of-array-except-self/gitsunmin.ts
new file mode 100644
index 000000000..3d06fa5c3
--- /dev/null
+++ b/product-of-array-except-self/gitsunmin.ts
@@ -0,0 +1,23 @@
+/**
+ * https://leetcode.com/problems/product-of-array-except-self
+ * time complexity : O(n)
+ * space complexity : O(1)
+ */
+function productExceptSelf(nums: number[]): number[] {
+    const n = nums.length;
+    const answer = new Array(n).fill(1);
+
+    let leftProduct = 1;
+    for (let i = 0; i < n; i++) {
+        answer[i] = leftProduct;
+        leftProduct *= nums[i];
+    }
+
+    let rightProduct = 1;
+    for (let i = n - 1; i >= 0; i--) {
+        answer[i] *= rightProduct;
+        rightProduct *= nums[i];
+    }
+
+    return answer;
+}
diff --git a/two-sum/gitsunmin.ts b/two-sum/gitsunmin.ts
new file mode 100644
index 000000000..23863c12d
--- /dev/null
+++ b/two-sum/gitsunmin.ts
@@ -0,0 +1,16 @@
+/**
+ * https://leetcode.com/problems/two-sum
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+function twoSum(nums: number[], target: number): number[] {
+    const m = new Map<number, number>();
+
+    for (let i = 0; i < nums.length; i++) {
+        if (m.has(nums[i]))
+            return [m.get(nums[i]), i];
+        m.set(target - nums[i], i);
+    }
+
+    return [-1, -1];
+};