diff --git a/merge-intervals/juhui-jeong.java b/merge-intervals/juhui-jeong.java
new file mode 100644
index 0000000000..ec92ae8a66
--- /dev/null
+++ b/merge-intervals/juhui-jeong.java
@@ -0,0 +1,28 @@
+/*
+ * 시간 복잡도: O(nlogn)
+ * 공간 복잡도: O(n)
+ */
+class Solution {
+    public int[][] merge(int[][] intervals) {
+        Arrays.sort(intervals, (a,b) -> {
+            if (a[0] != b[0]) return Integer.compare(a[0], b[0]);
+            return Integer.compare(a[1], b[1]);
+        });
+
+        List<int[]> merged = new ArrayList<>();
+        int[] cur = intervals[0];
+        merged.add(cur);
+
+        for (int i = 1; i < intervals.length; i++) {
+            int[] next = intervals[i];
+
+            if(cur[1] >= next[0]) {
+                cur[1] = Math.max(cur[1], next[1]);
+            } else {
+                cur = next;
+                merged.add(cur);
+            }
+        }
+        return merged.toArray(new int[merged.size()][]);
+    }
+}
diff --git a/missing-number/juhui-jeong.java b/missing-number/juhui-jeong.java
new file mode 100644
index 0000000000..6762315cc9
--- /dev/null
+++ b/missing-number/juhui-jeong.java
@@ -0,0 +1,31 @@
+/*
+ * 시간 복잡도: O(n)
+ * 공간 복잡도: O(1)
+ */
+class Solution {
+    public int missingNumber(int[] nums) {
+        int n = nums.length;
+        long expected = n * (n + 1) / 2;
+        long actual = 0;
+        for (int x : nums) {
+            actual += x;
+        }
+        return (int) (expected - actual);
+    }
+}
+
+/*
+ * 시간 복잡도: O(n log n)
+ * 공간 복잡도: O(1)
+ */
+class Solution {
+    public int missingNumber(int[] nums) {
+        Arrays.sort(nums);
+        for (int i = 0; i < nums.length; i++) {
+            if (i != nums[i]) {
+                return i;
+            }
+        }
+        return nums.length;
+    }
+}
diff --git a/reorder-list/juhui-jeong.java b/reorder-list/juhui-jeong.java
new file mode 100644
index 0000000000..44d8bcfb50
--- /dev/null
+++ b/reorder-list/juhui-jeong.java
@@ -0,0 +1,33 @@
+/*
+
+*/
+class Solution {
+    public void reorderList(ListNode head) {
+        List<ListNode> list = new ArrayList<>();
+
+        while(head != null) {
+            list.add(head);
+            head= head.next;
+        }
+
+        int i = 0;
+        int j = list.size() -1;
+        while(i < j) {
+            ListNode left = list.get(i);
+            ListNode right = list.get(j);
+
+            left.next = right;
+            i++;
+
+            if (i == j) {
+                right.next = null;
+                break;
+            }
+
+            right.next = list.get(i);
+            j--;
+        }
+
+        list.get(i).next = null;
+    }
+}