add new solutions

solutions/4/q401/solution.go

@@ -0,0 +1,45 @@
+package q401
+
+import "fmt"
+
+func bin2time(leds []uint8) (string, bool) {
+	hour := leds[0]<<3 + leds[1]<<2 + leds[2]<<1 + leds[3]
+	minute := leds[4]<<5 +
+		leds[5]<<4 +
+		leds[6]<<3 +
+		leds[7]<<2 +
+		leds[8]<<1 +
+		leds[9]
+	if hour >= 12 || minute >= 60 {
+		return "", false
+	}
+	return fmt.Sprintf("%d:%02d", hour, minute), true
+}
+
+func iterate(remaining, offset int, leds []uint8, ret *[]string) {
+	if 10-offset < remaining {
+		return
+	}
+	if remaining == 0 {
+		if r, ok := bin2time(leds); ok {
+			*ret = append(*ret, r)
+		}
+		return
+	}
+
+	leds[offset] = 1
+	iterate(remaining-1, offset+1, leds, ret)
+	leds[offset] = 0
+	iterate(remaining, offset+1, leds, ret)
+}
+
+func readBinaryWatch(turnedOn int) []string {
+	if turnedOn >= 9 {
+		return nil
+	}
+	ret := []string{}
+	iterate(turnedOn, 0, make([]uint8, 10), &ret)
+	return ret
+}
+
+var _ = readBinaryWatch