Simple solution based on binary search. However, I get an compile error says "Line 45: undefined: DriverSolution"

```
func hIndex(citations []int) int {
n := len(citations)
low := 0
high := n - 1
for low <= high {
mid := low + (high-low)/2
if citations[mid] == n-mid {
return n - mid
} else if citations[mid] > n-mid {
high = mid - 1
} else {
low = mid + 1
}
}
return n - low
}
```