diff --git a/contains-duplicate/YoungSeok-Choi.java b/contains-duplicate/YoungSeok-Choi.java
new file mode 100644
index 000000000..829ab78e2
--- /dev/null
+++ b/contains-duplicate/YoungSeok-Choi.java
@@ -0,0 +1,19 @@
+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+    // 시간복잡도 O(n)
+    public boolean containsDuplicate(int[] nums) {
+        Map<Integer, Boolean> dupMap = new HashMap<>();
+
+        for(int i = 0; i < nums.length; i++) {
+            if(dupMap.containsKey(nums[i])) {
+                return true;
+            }
+
+            dupMap.put(nums[i], true);
+        }
+
+        return false;
+    }
+}
diff --git a/house-robber/YoungSeok-Choi.java b/house-robber/YoungSeok-Choi.java
new file mode 100644
index 000000000..ba78f1f01
--- /dev/null
+++ b/house-robber/YoungSeok-Choi.java
@@ -0,0 +1,84 @@
+import java.util.HashMap;
+import java.util.Map;
+
+// 시간복잡도: O(n)
+// TODO: DP 방식으로 풀어보기
+class Solution {
+    public Map<Integer, Integer> robMap = new HashMap<>();
+    public int rob(int[] nums) {
+        return dfs(nums, 0);
+    }
+
+    public int dfs(int[] nums, int index) {
+        if(nums.length == 0) {
+            return 0;
+        }
+
+        if(index >= nums.length) {
+            return 0;
+        }
+
+        // 이미 털었던 집이라면, 해
+        if(robMap.containsKey(index)) {
+            return robMap.get(index);
+        }
+
+        // 이번 집을 털게되는 경우
+        int robThis = nums[index] + dfs(nums, index + 2);
+
+        // 이번 집을 털지않고 건너뛰는 경우,.
+        int skipThis = dfs(nums, index + 1);
+
+        robMap.put(index, Math.max(robThis, skipThis));
+
+        return robMap.get(index);
+    }
+}
+
+// TODO: 비효율적으로 작성한 알고리즘의 동작 방식을 도식화 해서 그려보기.
+// NOTE: dfs를 사용한 완전탐색
+// 탐색 방식이 매우 비효율적이라, 정답은 맞추지만 N이 커지면 시간초과
+// 시간복잡도: O(2^n) + alpha(중복탐색)
+class WrongSolution {
+    public boolean[] visit;
+    public int mx = -987654321;
+    public int curSum = 0;
+
+    public int rob(int[] nums) {
+        if(nums.length == 1) {
+            return nums[0];
+        }
+
+        visit = new boolean[nums.length];
+        dfs(nums, 0);
+        dfs(nums, 1);
+
+        return mx;
+    }
+
+    public void dfs(int[] arr, int idx) {
+        int len = arr.length;
+        int prevIdx = idx - 1;
+        int nextIdx = idx + 1;
+        
+
+        if(idx == 0) {
+            if(visit[idx]) return;
+        } else {
+            if(idx >= len || visit[idx] || visit[prevIdx]) {                
+                return;
+            }
+        }
+
+        visit[idx] = true;
+        curSum += arr[idx];
+        mx = Math.max(mx, curSum);
+
+        for(int i = idx; i < len; i++) {
+            dfs(arr, i);
+        }        
+
+        visit[idx] = false;
+        curSum -= arr[idx];
+    }
+}
diff --git a/longest-consecutive-sequence/YoungSeok-Choi.java b/longest-consecutive-sequence/YoungSeok-Choi.java
new file mode 100644
index 000000000..129633de9
--- /dev/null
+++ b/longest-consecutive-sequence/YoungSeok-Choi.java
@@ -0,0 +1,34 @@
+import java.util.Arrays;
+
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        int curSeq = 1;
+        int maxSeq = -987654321;
+
+        if(nums.length == 0) {
+            return 0;
+        }
+
+        Arrays.sort(nums);
+
+        int cur = nums[0];
+        for(int i = 1; i < nums.length; i++) {
+            if(cur == nums[i]) {
+                continue;
+            }
+
+            if(cur < nums[i] && Math.abs(nums[i] - cur) == 1) {
+                curSeq++;
+                cur = nums[i];
+                continue;
+            }
+
+            // NOTE: 수열의 조건이 깨졌을 때
+            maxSeq = Math.max(maxSeq, curSeq);
+            curSeq = 1;
+            cur = nums[i];
+        }
+
+        return Math.max(maxSeq, curSeq);
+    }
+}
diff --git a/product-of-array-except-self/YoungSeok-Choi.java b/product-of-array-except-self/YoungSeok-Choi.java
new file mode 100644
index 000000000..06886f67f
--- /dev/null
+++ b/product-of-array-except-self/YoungSeok-Choi.java
@@ -0,0 +1,47 @@
+class Solution {
+    // 시간복잡도: O(3n) -> O(n)
+    public int[] productExceptSelf(int[] nums) {
+
+        int zeroCount = 0;
+        int[] result = new int[nums.length];
+        int productExceptZero = 1;
+        int zeroIdx = 0;
+
+        for(int i = 0; i < nums.length; i++) {
+            if(nums[i] == 0) {
+                zeroCount++;
+            }
+        }
+
+        // NOTE: 0이 두개 이상일 때,  모든 배열의 원소가 0;
+        if(zeroCount >= 2) {
+            return result;
+        }
+
+        // NOTE: 0이 1개일 때, 0인 index만을 제외하고 모두 곱
+        if(zeroCount == 1) {
+            for(int i = 0; i < nums.length; i++) {
+                if(nums[i] == 0) {
+                    zeroIdx = i;
+                    continue;
+                }
+                productExceptZero *= nums[i];
+            }
+            
+            result[zeroIdx] = productExceptZero;
+            return result;
+        }
+
+        // NOTE: 0이 없을 때 모든수를 곱한 뒤 idx를 나누기.
+        for(int i = 0; i < nums.length; i++) {
+            productExceptZero *= nums[i];
+        }
+
+        for(int i = 0; i < nums.length; i++) {
+            int copy = productExceptZero;
+            result[i] = copy / nums[i];
+        }
+
+        return result;
+    }
+}
diff --git a/two-sum/YoungSeok-Choi.java b/two-sum/YoungSeok-Choi.java
new file mode 100644
index 000000000..09e8d136a
--- /dev/null
+++ b/two-sum/YoungSeok-Choi.java
@@ -0,0 +1,23 @@
+// time complexity: O(n);
+// 특정 nums[i] 가 target이 되기위한 보수 (target - nums[i])를 candidate Map에서 찾으면 종료
+
+import java.util.HashMap;
+import java.util.Map;
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        Map<Integer, Integer> candidate = new HashMap<>();
+
+        for(int i = 0; i < nums.length; i++) {
+
+            int diff = target - nums[i];
+
+            if(candidate.containsKey(diff)) {
+                return new int[] { candidate.get(diff), i };
+            }
+
+            candidate.put(nums[i], i);
+        }
+        return new int[0];
+    }
+}