examples: add euler.v

examples/euler.v

@@ -0,0 +1,18 @@
+// This example shows 2 solutions for https://projecteuler.net/problem=1 :
+fn gauss_sum(n int) fn (int) int {
+	return fn [n] (m int) int {
+		return m * ((n - 1) / m) * ((n - 1) / m + 1) / 2
+	}
+}
+
+gs := gauss_sum(1000)
+println('O(1) arithmetic progression sum: ${gs(3) + gs(5) - gs(15)}')
+
+// A brute force solution, by checking every n in the range:
+mut sum := 0
+for n in 1 .. 1000 {
+	if n % 3 == 0 || n % 5 == 0 {
+		sum += n
+	}
+}
+println('O(n) brute force calculated sum: ${sum}')