diff --git a/find-median-from-data-stream/HC-kang.ts b/find-median-from-data-stream/HC-kang.ts
new file mode 100644
index 000000000..091263023
--- /dev/null
+++ b/find-median-from-data-stream/HC-kang.ts
@@ -0,0 +1,164 @@
+/**
+ * https://leetcode.com/problems/find-median-from-data-stream
+ * array with binary search
+ */
+class MedianFinder {
+  // S.C. O(n)
+  private nums: number[] = [];
+
+  // T.C. O(n)
+  addNum(num: number): void {
+    if (this.nums.length === 0) {
+      this.nums.push(num);
+      return;
+    } else {
+      this.putNumWithBinarySearch(num);
+    }
+  }
+
+  private putNumWithBinarySearch(num: number): void {
+    let left = 0;
+    let right = this.nums.length - 1;
+
+    while (left <= right) {
+      const mid = Math.floor((left + right) / 2);
+      if (this.nums[mid] === num) {
+        this.nums.splice(mid, 0, num);
+        return;
+      } else if (this.nums[mid] < num) {
+        left = mid + 1;
+      } else {
+        right = mid - 1;
+      }
+    }
+
+    // T.C. O(n)
+    this.nums.splice(left, 0, num);
+  }
+
+  // T.C. O(1)
+  findMedian(): number {
+    const len = this.nums.length;
+
+    if (len % 2 === 1) {
+      return this.nums[Math.floor(len / 2)];
+    } else {
+      return (this.nums[len / 2] + this.nums[len / 2 - 1]) / 2;
+    }
+  }
+}
+
+/**
+ * heap...
+ * TL;DR
+ */
+class MedianFinder {
+  // S.C. O(n)
+  private smallHalf: MaxHeap = new MaxHeap();
+  private largeHalf: MinHeap = new MinHeap();
+
+  // T.C. O(log n)
+  addNum(num: number): void {
+    this.smallHalf.push(num);
+    this.largeHalf.push(this.smallHalf.pop()!);
+
+    if (this.smallHalf.size() < this.largeHalf.size()) {
+      this.smallHalf.push(this.largeHalf.pop()!);
+    }
+  }
+
+  // T.C. O(1)
+  findMedian(): number {
+    if (this.smallHalf.size() === this.largeHalf.size()) {
+      return (this.smallHalf.peek()! + this.largeHalf.peek()!) / 2;
+    } else {
+      return this.smallHalf.peek()!;
+    }
+  }
+}
+
+class MinHeap {
+  protected heap: number[] = [];
+
+  push(val: number): void {
+    this.heap.push(val);
+    this.heapifyUp();
+  }
+
+  pop(): number | undefined {
+    if (this.heap.length === 0) return undefined;
+    if (this.heap.length === 1) return this.heap.pop();
+
+    const result = this.heap[0];
+    this.heap[0] = this.heap.pop()!;
+    this.heapifyDown();
+    return result;
+  }
+
+  peek(): number | undefined {
+    return this.heap[0];
+  }
+
+  size(): number {
+    return this.heap.length;
+  }
+
+  private heapifyUp(): void {
+    let index = this.heap.length - 1;
+    while (index > 0) {
+      const parentIndex = Math.floor((index - 1) / 2);
+      if (this.heap[parentIndex] <= this.heap[index]) break;
+
+      this.swap(parentIndex, index);
+      index = parentIndex;
+    }
+  }
+
+  private heapifyDown(): void {
+    let index = 0;
+    while (true) {
+      let smallest = index;
+      const leftChild = 2 * index + 1;
+      const rightChild = 2 * index + 2;
+
+      if (
+        leftChild < this.heap.length &&
+        this.heap[leftChild] < this.heap[smallest]
+      ) {
+        smallest = leftChild;
+      }
+
+      if (
+        rightChild < this.heap.length &&
+        this.heap[rightChild] < this.heap[smallest]
+      ) {
+        smallest = rightChild;
+      }
+
+      if (smallest === index) break;
+
+      this.swap(index, smallest);
+      index = smallest;
+    }
+  }
+
+  protected swap(i: number, j: number): void {
+    [this.heap[i], this.heap[j]] = [this.heap[j], this.heap[i]];
+  }
+}
+
+class MaxHeap extends MinHeap {
+  push(val: number): void {
+    super.push(-val);
+  }
+
+  pop(): number | undefined {
+    const val = super.pop();
+    return val === undefined ? undefined : -val;
+  }
+
+  peek(): number | undefined {
+    const val = super.peek();
+    return val === undefined ? undefined : -val;
+  }
+}
diff --git a/house-robber/HC-kang.ts b/house-robber/HC-kang.ts
new file mode 100644
index 000000000..4e75f45bb
--- /dev/null
+++ b/house-robber/HC-kang.ts
@@ -0,0 +1,15 @@
+/**
+ * https://leetcode.com/problems/house-robber
+ * T.C. O(n)
+ * S.C. O(1)
+ */
+function rob(nums: number[]): number {
+  let prev = 0;
+  let curr = 0;
+  for (let i = 0; i < nums.length; i++) {
+    const temp = curr;
+    curr = Math.max(prev + nums[i], curr);
+    prev = temp;
+  }
+  return curr;
+}
diff --git a/lowest-common-ancestor-of-a-binary-search-tree/HC-kang.ts b/lowest-common-ancestor-of-a-binary-search-tree/HC-kang.ts
new file mode 100644
index 000000000..f6e1c0533
--- /dev/null
+++ b/lowest-common-ancestor-of-a-binary-search-tree/HC-kang.ts
@@ -0,0 +1,52 @@
+class TreeNode {
+  val: number;
+  left: TreeNode | null;
+  right: TreeNode | null;
+  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.left = left === undefined ? null : left;
+    this.right = right === undefined ? null : right;
+  }
+}
+
+/**
+ * https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/description
+ * T.C. O(log n)
+ * S.C. O(log n)
+ */
+function lowestCommonAncestor(
+  root: TreeNode | null,
+  p: TreeNode | null,
+  q: TreeNode | null
+): TreeNode | null {
+  if (!root) return null;
+  if (root.val > p!.val && root.val > q!.val) {
+    return lowestCommonAncestor(root.left, p, q);
+  } else if (root.val < p!.val && root.val < q!.val) {
+    return lowestCommonAncestor(root.right, p, q);
+  } else {
+    return root;
+  }
+}
+
+/**
+ * iterative
+ * T.C. O(log n)
+ * S.C. O(1)
+ */
+function lowestCommonAncestor(
+  root: TreeNode | null,
+  p: TreeNode | null,
+  q: TreeNode | null
+): TreeNode | null {
+  while (root) {
+    if (root.val > p!.val && root.val > q!.val) {
+      root = root.left;
+    } else if (root.val < p!.val && root.val < q!.val) {
+      root = root.right;
+    } else {
+      return root;
+    }
+  }
+  return null;
+}
diff --git a/meeting-rooms/HC-kang.ts b/meeting-rooms/HC-kang.ts
new file mode 100644
index 000000000..9fe1b2a04
--- /dev/null
+++ b/meeting-rooms/HC-kang.ts
@@ -0,0 +1,12 @@
+/**
+ * https://leetcode.com/problems/meeting-rooms
+ * T.C. O(nlogn)
+ * S.C. O(1)
+ */
+function canAttendMeetings(intervals: number[][]): boolean {
+  intervals.sort((a, b) => a[0] - b[0]);
+  for (let i = 0; i < intervals.length - 1; i++) {
+    if (intervals[i][1] > intervals[i + 1][0]) return false;
+  }
+  return true;
+}
diff --git a/non-overlapping-intervals/HC-kang.ts b/non-overlapping-intervals/HC-kang.ts
new file mode 100644
index 000000000..30bf802c1
--- /dev/null
+++ b/non-overlapping-intervals/HC-kang.ts
@@ -0,0 +1,29 @@
+/**
+ * https://leetcode.com/problems/non-overlapping-intervals
+ * T.C. O(n logn)
+ * S.C. O(1)
+ * 
+ * [[1,2],[2,3],[3,4],[1,3]] =(sort by end)=> [[1,2],[2,3],[1,3],[3,4]]
+ * 
+ *    0 1 2 3 4...
+ *      [=)      pass
+ *        [=)    pass
+ *      [===)    count++
+ *      ^^^
+ *          [=)  pass
+ */
+function eraseOverlapIntervals(intervals: number[][]): number {
+  intervals.sort((a, b) => a[1] - b[1]);
+
+  let count = 0;
+  let end = -Infinity;
+
+  for (let i = 0; i < intervals.length; i++) {
+    if (intervals[i][0] < end) { // 
+      count++;
+    } else {
+      end = intervals[i][1];
+    }
+  }
+  return count;
+}