valid-mountain-array.md

Code

func validMountainArray(arr []int) bool {
	if len(arr) < 3 {
		return false
	}

	n := len(arr)

	i := 0
	for i = 0; i < n-1; i++ {
		if arr[i] >= arr[i+1] {
			break
		}
	}

	if i == 0 {
		return false
	}

	j := i
	for j = i + 1; j < n; j++ {
		if arr[j] >= arr[j-1] {
			return false
		}
	}

	if j == i+1 {
		return false
	}

	return true
}

Solution in mind

• Solution is a simple one pass traversal of the array.

• As we climb p the array we ensure that arr[i+1] >= arr[i]. When this is violated, we reach a peak.

• We must ensure the peak is not at the very beginning (i == 0), if it was, we never climbed up the mountain.

• After reaching peak, we begin to climb down, while ensuring the condition arr[i] < arr[i-1], if not, we either have a rise or flat zone in the mountain.

• After descending from the peak, we ensure we have actually moved downwards and not stayed at the peak (j == i+1). This acts as a check to ensure the peak is not at the very end of the array too.