added java solution for Day008 that has O(1) time and space complexity

solutions/015.md

@@ -65,6 +65,32 @@ function sumOfMultiples(n: number): number {
 }
 ```
 
+
+#### Java (with O(1) complexity solution)
+```java
+class Day008 {
+    /* 
+        Calculate sum of multiples using arithmetic progression technique
+     */
+    public int sumOfMultiples(int n) {
+        return calcArithmetic(3, n) + calcArithmetic(5, n) + calcArithmetic(7, n) + calcArithmetic(3 * 5 * 7, n)
+                - calcArithmetic(3 * 5, n) - calcArithmetic(3 * 7, n) - calcArithmetic(7 * 5, n);
+    }
+
+    /*
+        Return arithmetic of numbers starting from a1 with a1 difference up to num using
+        the formula: s = n/2 * (a1 + an) where
+        - a1: first term
+        - an: last term
+        - n: number of terms
+     */
+    public int calcArithmetic(int a1, int num) {
+        int n = num / a1;
+        int an = n * a1;
+        return n * (a1 + an) / 2;
+    }
+}
+```
 The given solution solves the problem by performing the following steps:
 1. Initialize a variable `total` to 0 to keep track of the sum.
 2. Iterate over the range from 1 to `n+1`.