diff --git a/problem-1/problem-1.test.js b/problem-1/problem-1.test.js
index c775a02..2188a95 100644
--- a/problem-1/problem-1.test.js
+++ b/problem-1/problem-1.test.js
@@ -1,13 +1,27 @@
-const solution = (numbers) => {
+const solution = (numbers) => (numbers.length ? numbers.reduce((acc, curr) => acc + curr) : 0);
+
+const solution2 = (numbers) => {
+  let acc = 0;
+
+  while (numbers.length > 0) {
+    acc += numbers.pop();
+  }
+
+  return acc;
 };
 
 test('빈 배열은 0을 반환한다', () => {
   expect(solution([])).toBe(0);
+
+  expect(solution2([])).toBe(0);
 });
 
 test('배열의 합을 반환한다', () => {
   expect(solution([1, 2, 3, 4])).toBe(10);
   expect(solution([-1, 3, 8, 9, 10, 11])).toBe(40);
+
+  expect(solution2([1, 2, 3, 4])).toBe(10);
+  expect(solution2([-1, 3, 8, 9, 10, 11])).toBe(40);
 });
 
 test('큰 배열이 입력으로 주어져도 RangeError를 던지지 않는다', () => {
@@ -15,4 +29,7 @@ test('큰 배열이 입력으로 주어져도 RangeError를 던지지 않는다'
 
   expect(() => solution(input))
     .not.toThrowError(new RangeError('Maximum call stack size exceeded'));
+
+  expect(() => solution2(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..e165589 100644
--- a/problem-2/problem-2.test.js
+++ b/problem-2/problem-2.test.js
@@ -1,4 +1,20 @@
 const solution = (n) => {
+  if (n < 0) return 0;
+  if (n < 2) return n;
+
+  let current = 2;
+  let a = 0;
+  let b = 1;
+  let sum = a + b;
+
+  while (current < n) {
+    current += 1;
+    a = b;
+    b = sum;
+    sum = a + b;
+  }
+
+  return sum;
 };
 
 test('음수가 주어지면 0을 반환한다', () => {
diff --git a/problem-3/problem-3.test.js b/problem-3/problem-3.test.js
index 7319eb8..cc449f4 100644
--- a/problem-3/problem-3.test.js
+++ b/problem-3/problem-3.test.js
@@ -1,4 +1,18 @@
 const solution = (n) => {
+  if (n === 0) {
+    return '0';
+  }
+
+  let memo = '';
+  let number = n;
+
+  while (number > 1) {
+    const remainder = number % 2;
+    memo = remainder + memo;
+    number = Math.floor(number / 2);
+  }
+
+  return `1${memo}`;
 };
 
 test('이진수 문자열을 반환한다', () => {
diff --git a/problem-4/problem-4.test.js b/problem-4/problem-4.test.js
index 19f5cb0..cf6a982 100644
--- a/problem-4/problem-4.test.js
+++ b/problem-4/problem-4.test.js
@@ -1,5 +1,11 @@
-const solution = () => {
-};
+const solution = (binNum) => binNum
+  .split('')
+  .reverse()
+  .map((x, idx) => {
+    if (x === '0') { return 0; }
+    return (2 ** idx);
+  })
+  .reduce((acc, curr) => acc + curr);
 
 test('10진수 숫자를 반환한다', () => {
   expect(solution('0')).toBe(0);
diff --git a/problem-5/problem-5.test.js b/problem-5/problem-5.test.js
index 20a8fab..479fdf0 100644
--- a/problem-5/problem-5.test.js
+++ b/problem-5/problem-5.test.js
@@ -1,4 +1,6 @@
-const solution = () => {
+const solution = (a, b) => {
+  const remainder = a % b;
+  return remainder === 0 ? b : solution(b, remainder);
 };
 
 test('최대 공약수를 반환한다', () => {
diff --git a/problem-6/problem-6.test.js b/problem-6/problem-6.test.js
index 059e2b8..5949cc1 100644
--- a/problem-6/problem-6.test.js
+++ b/problem-6/problem-6.test.js
@@ -1,4 +1,22 @@
 const solution = (n) => {
+  let a = 1;
+  if (n === 1) return a;
+
+  let b = 2;
+  if (n === 2) return b;
+
+  let count = 3;
+  let result = 4;
+
+  while (count < n) {
+    const temp = result;
+    result = a + b + result;
+    a = b;
+    b = temp;
+
+    count += 1;
+  }
+  return result;
 };
 
 test('계단에 오를 수 있는 가지 수를 반환한다', () => {