* Update SinglyLinkedList

* Refactor MergeSortedSinglyLinkedList to single file
* Create Problem09 and Problem10 in Project Euler

ProjectEuler/Problem09.java

@@ -0,0 +1,30 @@
+package ProjectEuler;
+
+/**
+ * A Pythagorean triplet is a set of three natural numbers, a < b < c, for which,
+ * <p>
+ * a^2 + b^2 = c^2
+ * For example, 3^2 + 4^2 = 9 + 16 = 25 = 5^2.
+ * <p>
+ * There exists exactly one Pythagorean triplet for which a + b + c = 1000.
+ * Find the product abc.
+ * <p>
+ * link: https://projecteuler.net/problem=9
+ */
+public class Problem09 {
+    public static void main(String[] args) {
+        assert solution1() == 31875000;
+    }
+
+    private static int solution1() {
+        for (int i = 0; i <= 300; ++i) {
+            for (int j = 0; j <= 400; ++j) {
+                int k = 1000 - i - j;
+                if (i * i + j * j == k * k) {
+                    return i * j * k;
+                }
+            }
+        }
+        return -1; /* should not happen */
+    }
+}