Added ProjectEuler

ProjectEuler/Problem01.java

@@ -0,0 +1,51 @@
+package ProjectEuler;
+
+/**
+ * If we list all the natural numbers below 10 that are multiples of 3 or 5,
+ * we get 3, 5, 6 and 9. The sum of these multiples is 23.
+ * <p>
+ * Find the sum of all the multiples of 3 or 5 below 1000.
+ * <p>
+ * Link: https://projecteuler.net/problem=1
+ */
+public class Problem01 {
+    public static void main(String[] args) {
+        int[][] testNumber = {
+                {3, 0},
+                {4, 3},
+                {10, 23},
+                {1000, 233168},
+                {-1, 0}
+        };
+
+        for (int[] ints : testNumber) {
+            assert solution1(ints[0]) == ints[1];
+            assert solution2(ints[0]) == ints[1];
+        }
+    }
+
+    private static int solution1(int n) {
+        int sum = 0;
+        for (int i = 3; i < n; ++i) {
+            if (i % 3 == 0 || i % 5 == 0) {
+                sum += i;
+            }
+        }
+        return sum;
+    }
+
+    private static int solution2(int n) {
+        int sum = 0;
+
+        int terms = (n - 1) / 3;
+        sum += terms * (6 + (terms - 1) * 3) / 2;
+
+        terms = (n - 1) / 5;
+        sum += terms * (10 + (terms - 1) * 5) / 2;
+
+        terms = (n - 1) / 15;
+        sum -= terms * (30 + (terms - 1) * 15) / 2;
+
+        return sum;
+    }
+}