* 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 */
+    }
+}

ProjectEuler/Problem10.java

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+package ProjectEuler;
+
+/**
+ * The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
+ * <p>
+ * Find the sum of all the primes below two million.
+ * <p>
+ * link: https://projecteuler.net/problem=10
+ */
+public class Problem10 {
+    public static void main(String[] args) {
+        long[][] testNumbers = {
+                {2000000, 142913828922L},
+                {10000, 5736396},
+                {5000, 1548136},
+                {1000, 76127},
+                {10, 17},
+                {7, 10}
+        };
+
+        for (long[] testNumber : testNumbers) {
+            assert solution1(testNumber[0]) == testNumber[1];
+        }
+    }
+
+    /***
+     * Checks if a number is prime or not
+     * @param n the number
+     * @return {@code true} if {@code n} is prime
+     */
+    private static boolean isPrime(int n) {
+        if (n == 2) {
+            return true;
+        }
+        if (n < 2 || n % 2 == 0) {
+            return false;
+        }
+        for (int i = 3, limit = (int) Math.sqrt(n); i <= limit; i += 2) {
+            if (n % i == 0) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private static long solution1(long n) {
+        long sum = 0;
+        for (int i = 2; i < n; ++i) {
+            if (isPrime(i)) {
+                sum += i;
+            }
+        }
+        return sum;
+    }
+}