diff --git a/Miscellaneous/SOE/HappyNumber.java b/Miscellaneous/SOE/HappyNumber.java
new file mode 100644
index 0000000..4507bc9
--- /dev/null
+++ b/Miscellaneous/SOE/HappyNumber.java
@@ -0,0 +1,47 @@
+package SOE;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class HappyNumber {
+
+    // 計算各位數平方和
+    static int next(int n) {
+        int sum = 0;
+        while (n > 0) {
+            int d = n % 10;
+            sum += d * d;
+            n /= 10;
+        }
+        return sum;
+    }
+
+    // 遞迴版 isHappy()
+    static boolean isHappy(int n) {
+        return isHappyHelper(n, new HashSet<>());
+    }
+
+    // 遞迴輔助函式：傳遞 seen 集合
+    private static boolean isHappyHelper(int n, Set<Integer> seen) {
+        if (n == 1)
+            return true; // 終止條件：快樂數
+        if (seen.contains(n))
+            return false; // 終止條件：進入循環
+
+        seen.add(n);
+        return isHappyHelper(next(n), seen);
+    }
+
+    // 測試
+    public static void main(String[] args) {
+        int[] nums = {1, 7, 19, 20, 25, 4};
+
+        for (int n : nums) {
+            System.out.printf("%d -> %s%n", n, isHappy(n));
+        }
+    }
+}
+
+// Source: https://chatgpt.com/c/69104fa3-4828-8321-ade1-f983f1f8c14c
+
+