add new solutions

solutions/15/q1518/solution.go

@@ -0,0 +1,19 @@
+package q1518
+
+func numWaterBottles(numBottles int, numExchange int) int {
+	drink := 0
+	empty := 0
+	for {
+		drink += numBottles
+		empty += numBottles
+		if empty < numExchange {
+			break
+		}
+
+		numBottles = empty / numExchange
+		empty = empty % numExchange
+	}
+	return drink
+}
+
+var _ = numWaterBottles