diff --git a/find-median-from-data-stream/Tessa1217.java b/find-median-from-data-stream/Tessa1217.java
new file mode 100644
index 000000000..ff717faf6
--- /dev/null
+++ b/find-median-from-data-stream/Tessa1217.java
@@ -0,0 +1,46 @@
+import java.util.PriorityQueue;
+
+class MedianFinder {
+
+    // 작은 수 범위 저장하는 힙
+    private PriorityQueue<Integer> smallHeap;
+
+    // 큰 수 범위 저장하는 힙
+    private PriorityQueue<Integer> largeHeap;
+
+    public MedianFinder() {
+        smallHeap = new PriorityQueue<>((a, b) -> b - a);
+        largeHeap = new PriorityQueue<> ((a, b) -> a - b);
+    }
+
+    public void addNum(int num) {
+        // 작은 수 범위에 삽입
+        smallHeap.offer(num);
+        // 작은 수 범위에서 최댓값을 뽑아 큰 수 범위로 이동
+        largeHeap.offer(smallHeap.poll());
+
+        // 만약 작은 수 범위의 개수가 큰 수 범위보다 작다면
+        if (smallHeap.size() < largeHeap.size()) {
+            // 큰 수 범위에서 최솟값을 뽑아 작은 수 범위로 이동
+            smallHeap.offer(largeHeap.poll());
+        }
+    }
+
+    public double findMedian() {
+        // 짝수 개일 경우
+        if (smallHeap.size() == largeHeap.size()) {
+            // 작은 수 범위 힙의 최댓값 + 큰 수 범위 힙의 최솟값의 평균
+            return (smallHeap.peek() + largeHeap.peek()) / 2.0;
+        }
+        // 작은 수 범위 힙의 최댓값
+        return smallHeap.peek();
+    }
+}
+
+/**
+ * Your MedianFinder object will be instantiated and called as such:
+ * MedianFinder obj = new MedianFinder();
+ * obj.addNum(num);
+ * double param_2 = obj.findMedian();
+ */
+
diff --git a/insert-interval/Tessa1217.java b/insert-interval/Tessa1217.java
new file mode 100644
index 000000000..8e14916ab
--- /dev/null
+++ b/insert-interval/Tessa1217.java
@@ -0,0 +1,39 @@
+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+
+    // 시간, 공간복잡도: O(n)
+    public int[][] insert(int[][] intervals, int[] newInterval) {
+
+        // 병합된 interval 담는 list
+        List<int[]> modifyIntervals = new ArrayList<>();
+
+        int idx = 0;
+
+        // 병합 이전 구간
+        while (idx < intervals.length && intervals[idx][1] < newInterval[0]) {
+            modifyIntervals.add(intervals[idx]);
+            idx++;
+        }
+
+        // 병합이 필요한 구간 (newInterval과 겹치는 구간)
+        while (idx < intervals.length && intervals[idx][0] <= newInterval[1]) {
+            newInterval[0] = Math.min(intervals[idx][0], newInterval[0]);
+            newInterval[1] = Math.max(intervals[idx][1], newInterval[1]);
+            idx++;
+        }
+
+        // 최종 병합된 새로운 interval add        
+        modifyIntervals.add(newInterval);
+
+        // 병합 이후 구간
+        while (idx < intervals.length) {
+            modifyIntervals.add(intervals[idx]);
+            idx++;
+        }
+
+        return modifyIntervals.toArray(new int[modifyIntervals.size()][2]);
+    }
+}
+
diff --git a/kth-smallest-element-in-a-bst/Tessa1217.java b/kth-smallest-element-in-a-bst/Tessa1217.java
new file mode 100644
index 000000000..452fd995a
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/Tessa1217.java
@@ -0,0 +1,51 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+
+    private int count = 0;
+
+    private int kthSmallValue = 0;
+
+    // 시간복잡도: O(k) (불균형 상태의 이진 트리일 경우 O(n))
+    public int kthSmallest(TreeNode root, int k) {
+        orderSearch(root, k);
+        return kthSmallValue;
+    }
+
+    // In Order Search
+    private void orderSearch(TreeNode node, int k) {
+
+        if (node == null) {
+            return;
+        }
+
+        // HINT => utilize the property of a BST => 좌측 리프 노드부터 탐색
+        orderSearch(node.left, k);
+
+        count++;
+
+        if (count == k) {
+            kthSmallValue = node.val;
+            return;
+        }
+
+        // search right side
+        orderSearch(node.right, k);
+
+    }
+
+}
+
diff --git a/lowest-common-ancestor-of-a-binary-search-tree/Tessa1217.java b/lowest-common-ancestor-of-a-binary-search-tree/Tessa1217.java
new file mode 100644
index 000000000..cc5d2dfb2
--- /dev/null
+++ b/lowest-common-ancestor-of-a-binary-search-tree/Tessa1217.java
@@ -0,0 +1,33 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+
+        if (root == null || root == p || root == q) {
+            return root;
+        }
+
+        // 재귀로 좌측과 우측 탐색
+        TreeNode left = lowestCommonAncestor(root.left, p, q);
+
+        TreeNode right = lowestCommonAncestor(root.right, p, q);
+
+        // 좌우 둘 다 null이 아니라면: 현재 root를 조상으로 하는 서브 트리에서 p와 q를 발견했음을 의미하므로 root가 공통 조상
+        if (left != null && right != null) {
+            return root;
+        } else if (left != null) {
+            return left;
+        } else {
+            return right;
+        }
+
+    }
+}
+
diff --git a/meeting-rooms/Tessa1217.java b/meeting-rooms/Tessa1217.java
new file mode 100644
index 000000000..3ad8cf994
--- /dev/null
+++ b/meeting-rooms/Tessa1217.java
@@ -0,0 +1,42 @@
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Definition of Interval:
+ * public class Interval {
+ *     int start, end;
+ *     Interval(int start, int end) {
+ *         this.start = start;
+ *         this.end = end;
+ *     }
+ * }
+ */
+
+public class Solution {
+    /**
+     * @param intervals: an array of meeting time intervals
+     * @return: if a person could attend all meetings
+     */
+
+    // 시간복잡도: O(n log n) - 정렬, 공간복잡도: O(1)
+    public boolean canAttendMeetings(List<Interval> intervals) {
+        // Write your code here
+
+        if (intervals == null || intervals.isEmpty()) return true;
+
+        Collections.sort(intervals, (i1, i2) -> i1.start - i2.start);
+
+        Interval previous = intervals.get(0);
+
+        for (int i = 1; i < intervals.size(); i++) {
+            Interval current = intervals.get(i);
+            if (previous.end > current.start) {
+                return false;
+            }
+            previous = current;
+        }
+
+        return true;
+    }
+}
+