diff --git a/problem-1/problem-1.test.js b/problem-1/problem-1.test.js
index c775a02..166fd25 100644
--- a/problem-1/problem-1.test.js
+++ b/problem-1/problem-1.test.js
@@ -1,5 +1,36 @@
-const solution = (numbers) => {
-};
+// const solutionEasy = (numbers) => {
+//   let acc = 0;
+
+//   numbers.forEach((cur) => {
+//     acc += cur;
+//   });
+
+//   return acc;
+// };
+
+// const solutionRecursive = (numbers) => {
+//   if (numbers.length === 0) {
+//     return 0;
+//   }
+
+//   return numbers.pop() + solutionRecursive(numbers);
+// };
+
+// const solutionLoop = (numbers) => {
+//   let acc = 0;
+
+//   while (true) {
+//     if (numbers.length === 0) {
+//       break;
+//     }
+
+//     acc += numbers.pop();
+//   }
+
+//   return acc;
+// };
+
+const solution = (numbers) => numbers.reduce((acc, cur) => acc + cur, 0);
 
 test('빈 배열은 0을 반환한다', () => {
   expect(solution([])).toBe(0);
@@ -13,6 +44,7 @@ test('배열의 합을 반환한다', () => {
 test('큰 배열이 입력으로 주어져도 RangeError를 던지지 않는다', () => {
   const input = Array.from({ length: 10000 }, (_, i) => i + 1);
 
-  expect(() => solution(input))
-    .not.toThrowError(new RangeError('Maximum call stack size exceeded'));
+  expect(() => solution(input)).not.toThrowError(
+    new RangeError('Maximum call stack size exceeded'),
+  );
 });
diff --git a/problem-2/problem-2.test.js b/problem-2/problem-2.test.js
index 458a6b8..6cf9cbe 100644
--- a/problem-2/problem-2.test.js
+++ b/problem-2/problem-2.test.js
@@ -1,4 +1,47 @@
+// const solutionRecursive = (n) => {
+//   if (n <= 0) {
+//     return 0;
+//   }
+
+//   if (n === 1) {
+//     return 1;
+//   }
+
+//   return solutionRecursive(n - 2) + solutionRecursive(n - 1);
+// };
+
+// const solutionTailRecursive = (n, prv = 0, cur = 1) => {
+//   if (n <= 0) {
+//     return prv;
+//   }
+
+//   if (n === 1) {
+//     return cur;
+//   }
+
+//   return solutionTailRecursive(n - 1, cur, prv + cur);
+// };
+
 const solution = (n) => {
+  let prv = 0;
+  let cur = 1;
+  let tmp = 1;
+  let counter = n;
+
+  while (true) {
+    if (counter <= 0) {
+      return prv;
+    }
+
+    if (counter === 1) {
+      return cur;
+    }
+
+    tmp = prv + cur;
+    prv = cur;
+    cur = tmp;
+    counter -= 1;
+  }
 };
 
 test('음수가 주어지면 0을 반환한다', () => {
diff --git a/problem-3/problem-3.test.js b/problem-3/problem-3.test.js
index 7319eb8..39f7734 100644
--- a/problem-3/problem-3.test.js
+++ b/problem-3/problem-3.test.js
@@ -1,4 +1,13 @@
-const solution = (n) => {
+const solution = (n, result = '') => {
+  if (n === 0 && result === '') {
+    return '0';
+  }
+
+  if (n === 0) {
+    return result;
+  }
+
+  return solution(Math.floor(n / 2), (n % 2) + result);
 };
 
 test('이진수 문자열을 반환한다', () => {
diff --git a/problem-6/problem-6.test.js b/problem-6/problem-6.test.js
index 059e2b8..f411237 100644
--- a/problem-6/problem-6.test.js
+++ b/problem-6/problem-6.test.js
@@ -1,4 +1,37 @@
-const solution = (n) => {
+// const solutionRecursive = (n) => {
+//   if (n === 1) {
+//     return 1;
+//   }
+
+//   if (n === 2) {
+//     return 2;
+//   }
+
+//   if (n === 3) {
+//     return 4;
+//   }
+
+//   return solutionRecursive(n - 3) + solutionRecursive(n - 2) + solutionRecursive(n - 1);
+// };
+
+const solution = (n, memo = []) => {
+  if (n === 1) {
+    return 1;
+  }
+
+  if (n === 2) {
+    return 2;
+  }
+
+  if (n === 3) {
+    return 4;
+  }
+
+  if (!memo[n]) {
+    return solution(n - 3, memo) + solution(n - 2, memo) + solution(n - 1, memo);
+  }
+
+  return memo[n];
 };
 
 test('계단에 오를 수 있는 가지 수를 반환한다', () => {