diff --git a/contains-duplicate/essaysir.java b/contains-duplicate/essaysir.java
new file mode 100644
index 0000000000..bf0aa00b57
--- /dev/null
+++ b/contains-duplicate/essaysir.java
@@ -0,0 +1,12 @@
+import java.util.*;
+
+class Solution {
+	public boolean containsDuplicate(int[] nums) {
+		Set<Integer> sets = new HashSet<>();
+		for ( int i = 0; i < nums.length; i++){
+			boolean added = sets.add(nums[i]);
+			if (!added) return true;
+		}
+		return false;
+	}
+}
diff --git a/house-robber/essaysir.java b/house-robber/essaysir.java
new file mode 100644
index 0000000000..eef41621b1
--- /dev/null
+++ b/house-robber/essaysir.java
@@ -0,0 +1,15 @@
+class Solution {
+	public int rob(int[] nums) {
+		int prev = 0;  // dp[i-2]: 두 칸 전까지의 최대 금액
+		int curr = 0;  // dp[i-1]: 한 칸 전까지의 최대 금액
+
+		for (int num : nums) {
+			// 현재 집을 털 경우(prev + num) vs 안 털 경우(curr) 중 큰 값
+			int temp = Math.max(curr, prev + num);
+			prev = curr;
+			curr = temp;
+		}
+
+		return curr;
+	}
+}
diff --git a/longest-consecutive-sequence/essaysir.java b/longest-consecutive-sequence/essaysir.java
new file mode 100644
index 0000000000..e37b1ac54c
--- /dev/null
+++ b/longest-consecutive-sequence/essaysir.java
@@ -0,0 +1,23 @@
+import java.util.*;
+
+class Solution {
+    // TC: O(n)
+    // SC: O(n)
+    public int longestConsecutive(int[] nums) {
+        // 연속적인 수의 개수를 구한다.
+        Arrays.sort(nums);
+        int maxCount = 0;
+        int cur = 1;
+        for ( int i = 1 ; i < nums.length; i ++){
+            if (nums[i] == nums[i - 1]) {
+                continue;                       // 중복 건너뛰기
+            } else if (nums[i - 1] + 1 == nums[i]) {
+                cur += 1;
+            } else {
+                cur = 1;
+            }
+            maxCount = Math.max(maxCount, cur); // 매 스텝마다 갱신
+        }
+        return maxCount;
+    }
+}
diff --git a/top-k-frequent-elements/essaysir.java b/top-k-frequent-elements/essaysir.java
new file mode 100644
index 0000000000..856764a4b5
--- /dev/null
+++ b/top-k-frequent-elements/essaysir.java
@@ -0,0 +1,27 @@
+import java.util.*;
+
+class Solution {
+    // TC: O(n log n)
+    // SC: O(n)
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> count = new HashMap<>();
+        for ( int i = 0; i < nums.length; i++){
+            count.put(nums[i], count.getOrDefault(nums[i],0)+ 1);
+        }
+
+        PriorityQueue<Integer> heap = new PriorityQueue<>((a,b) -> count.get(a) - count.get(b));
+
+        for ( int key : count.keySet()){
+            heap.offer(key);
+            if ( heap.size() > k ){
+                heap.poll();
+            }
+        }
+
+        int[] result = new int[k];
+        for ( int i = 0; i < k; i++){
+            result[i] = heap.poll();
+        }
+        return result;
+    }
+}
diff --git a/two-sum/essaysir.java b/two-sum/essaysir.java
new file mode 100644
index 0000000000..3eb19ebda5
--- /dev/null
+++ b/two-sum/essaysir.java
@@ -0,0 +1,17 @@
+import java.util.*;
+
+class Solution {
+	public int[] twoSum(int[] nums, int target) {
+		// TWO Sum , 순서는 상관이 없음. NC2 를 계산 필요
+		int[] result = new int[2];
+		for ( int i = 0; i < nums.length; i++){
+			for ( int j = i + 1; j < nums.length; j++){
+				if ( target == nums[i] + nums[j]){
+					result[0] = i;
+					result[1] = j;
+				}
+			}
+		}
+		return result;
+	}
+}