Generate all possible combinations of `num`

indexes in the range from 0 to 10. For example, one possible combination of 5 indexes is `{0, 1, 4, 7, 8}`

, which is "3:25".

```
def readBinaryWatch(self, num):
watch = [1,2,4,8,1,2,4,8,16,32]
times = []
for leds in itertools.combinations(range(len(watch)), num):
h = sum(watch[i] for i in leds if i < 4)
m = sum(watch[i] for i in leds if i >= 4)
if h > 11 or m > 59: continue
times.append("{}:{:02d}".format(h, m))
return times
```