diff --git a/binary-tree-level-order-traversal/TonyKim9401.java b/binary-tree-level-order-traversal/TonyKim9401.java
new file mode 100644
index 000000000..127b79377
--- /dev/null
+++ b/binary-tree-level-order-traversal/TonyKim9401.java
@@ -0,0 +1,26 @@
+// TC: O(n)
+// need to visit all nodes
+// SC: O(n)
+// normally O(log n) required however, O(n) in the worst case
+class Solution {
+    private List<List<Integer>> output = new ArrayList<>();
+    public List<List<Integer>> levelOrder(TreeNode root) {
+        dfs(0, root);
+        return output;
+    }
+
+    private void dfs(int level, TreeNode node) {
+        if (node == null) return;
+
+        if (output.size() == level) {
+            List<Integer> inside = new ArrayList<>();
+            inside.add(node.val);
+            output.add(inside);
+        } else {
+            output.get(level).add(node.val);
+        }
+        level += 1;
+        dfs(level, node.left);
+        dfs(level, node.right);
+    }
+}
diff --git a/house-robber-ii/TonyKim9401.java b/house-robber-ii/TonyKim9401.java
new file mode 100644
index 000000000..87c047264
--- /dev/null
+++ b/house-robber-ii/TonyKim9401.java
@@ -0,0 +1,32 @@
+// TC: O(n)
+// visit all nums at least once
+// SC: O(1)
+// only constant memory space required
+class Solution {
+    public int rob(int[] nums) {
+        if (nums.length == 1) return nums[0];
+
+        int prev = 0;
+        int post = 0;
+        int output1 = 0;
+
+        for (int i = 0; i < nums.length - 1; i++) {
+            int temp = prev;
+            prev = Math.max(post + nums[i], prev);
+            post = temp;
+        }
+        output1 = prev;
+
+        prev = 0;
+        post = 0;
+        int output2 = 0;
+        for (int i = 1; i < nums.length; i++) {
+            int temp = prev;
+            prev = Math.max(post + nums[i], prev);
+            post = temp;
+        }
+        output2 = prev;
+
+        return Math.max(output1, output2);
+    }
+}
diff --git a/meeting-rooms-ii/TonyKim9401.java b/meeting-rooms-ii/TonyKim9401.java
new file mode 100644
index 000000000..e58e18540
--- /dev/null
+++ b/meeting-rooms-ii/TonyKim9401.java
@@ -0,0 +1,22 @@
+// TC: O(n)
+// visit all intervals to compare each of start time
+// SC: O(n)
+// PQ save all intervals in the worst case
+public class Solution {
+    public int minMeetingRooms(List<Interval> intervals) {
+        if (intervals == null || intervals.isEmpty()) return 0;
+
+        intervals.sort((a, b) -> a.start - b.start);
+
+        PriorityQueue<Integer> endTimes = new PriorityQueue<>();
+        endTimes.add(intervals.get(0).end);
+
+        for (int i = 1; i < intervals.size(); i++) {
+            Interval current = intervals.get(i);
+            if (current.start >= endTimes.peek()) endTimes.poll();
+            endTimes.add(current.end);
+        }
+
+        return endTimes.size();
+    }
+}
diff --git a/reverse-bits/TonyKim9401.java b/reverse-bits/TonyKim9401.java
new file mode 100644
index 000000000..a1d107579
--- /dev/null
+++ b/reverse-bits/TonyKim9401.java
@@ -0,0 +1,14 @@
+public class Solution {
+    public int reverseBits(int n) {
+        // time complexity O(1)
+        // space complexity O(1)
+        int output = 0;
+
+        for (int i = 0; i < Integer.SIZE; i++) {
+            output <<= 1;
+            output += n & 1;
+            n >>= 1;
+        }
+        return output;
+    }
+}