diff --git a/merge-intervals/Yiseull.java b/merge-intervals/Yiseull.java
new file mode 100644
index 0000000000..95e9869aa6
--- /dev/null
+++ b/merge-intervals/Yiseull.java
@@ -0,0 +1,16 @@
+class Solution {
+    public int[][] merge(int[][] intervals) {
+        Arrays.sort(intervals, (o1, o2) -> o1[0] - o2[0]);
+
+        int index = 0;
+        for (int i = 0; i < intervals.length; i++) {
+            if (intervals[index][1] < intervals[i][0]) {
+                intervals[++index] = intervals[i];
+            } else {
+                intervals[index][1] = Math.max(intervals[index][1], intervals[i][1]);
+            }
+        }
+
+        return Arrays.copyOf(intervals, index + 1);
+    }
+}
diff --git a/missing-number/Yiseull.java b/missing-number/Yiseull.java
new file mode 100644
index 0000000000..7cf15159b4
--- /dev/null
+++ b/missing-number/Yiseull.java
@@ -0,0 +1,13 @@
+class Solution {
+    public int missingNumber(int[] nums) {
+        int n = nums.length;
+        int expectedSum = n * (n + 1) / 2;
+        int actualSum = 0;
+
+        for (int num : nums) {
+            actualSum += num;
+        }
+
+        return expectedSum - actualSum;
+    }
+}