Where the climb turns into a descent
A pressure trace rises to a single peak and then falls away. The analysis wants the position of the peak, and the traces are long.
- The readings rise strictly to one peak, then fall strictly away.
- There is exactly one peak; it may be the first or the last reading.
- Return its position, counting from zero.
- An empty trace has no peak: return -1.
peakPosition(trace: list<int>) → int
Go needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
func peakPosition(trace []int) int {
}Worked examples
| Call | Result |
|---|---|
peakPosition([]int{1, 3, 5, 4, 2}) | 2 |
peakPosition([]int{1, 2, 3}) | 2 |
peakPosition([]int{9, 5, 1}) | 0 |
peakPosition([]int{7}) | 0 |
Hint
Compare a reading with the one after it. Still climbing means the peak is further right; already falling means it is here or to the left.
Reference solution in Go
func peakPosition(trace []int) int {
if len(trace) == 0 {
return -1
}
lo, hi := 0, len(trace)-1
for lo < hi {
mid := (lo + hi) / 2
if trace[mid] < trace[mid+1] {
lo = mid + 1
} else {
hi = mid
}
}
return lo
}