Hackerrank - Counting Valleys

Gary is an avid hiker. He tracks his hikes meticulously, paying close attention to small details like topography. During his last hike he took exactly n steps. For every step he took, he noted if it was an uphill, U , or a downhill, D step. Gary’s hikes start and end at sea level and each step up or down represents a 1 unit change in altitude. We define the following terms:

• A mountain is a sequence of consecutive steps above sea level, starting with a step up from sea level and ending with a step down to sea level.
• A valley is a sequence of consecutive steps below sea level, starting with a step down from sea level and ending with a step up to sea level.

Given Gary’s sequence of up and down steps during his last hike, find and print the number of valleys he walked through.

For example, if Gary’s path is s = [DDUUUUDD], he first enters a valley units deep. Then he climbs out an up onto a mountain 2 units high. Finally, he returns to sea level and ends his hike.

Function Description

Complete the countingValleys function in the editor below. It must return an integer that denotes the number of valleys Gary traversed.

countingValleys has the following parameter(s):

• n: the number of steps Gary takes
• s: a string describing his path

Input Format

The first line contains an integer n, the number of steps in Gary’s hike.
The second line contains a single string s, of n characters that describe his path.

Constraints

• 2 ≦ n ≦ 10⁶
• s[i] ∈ {U D}

Output Format

Print a single integer that denotes the number of valleys Gary walked through during his hike.

Sample Input

8
UDDDUDUU

Sample Output

1

Explanation

If we represent _ as sea level, a step up as /, and a step down as , Gary’s hike can be drawn as:

_/\      _
   \    /
    \/\/

He enters and leaves one valley.

최초 풀이

def countingValleys(n, s):
  # 문자열 동선을 정수 리스트로 변환
  s_list = []
  for i in s:
    if i == 'U':
      s_list.append(1)
    else:
      s_list.append(-1)
  s = s_list

  # 0과 0 사이(해수면과 해수면 사이) 값의 합을 구하기
  num_sum = 0
  sum_list = []
  for idx in range(len(s)):
    num_sum += s[idx]
    if num_sum == 0:
      num_sum -= s[idx]
      sum_list.append(num_sum)
      num_sum = 0

  # 음수가 몇 개 있는지(계곡에 몇 번 들어갔는지) 구해서 리턴
  cnt = 0
  for value in sum_list:
    if value < 0:
      cnt += 1
  return cnt

0은 해수면이므로 계곡을 몇 번 들어갔는지 알기 위헤서는 0과 0사이 숫자 합을 구한 후 음수가 몇 개 있는지 리턴하면 된다고 판단했다.

동선에서 U는 한 칸 위로 올라가는 개념이므로 +1이고 D는 -1이다. 문자열로 우선 주어진 동선을 정수 리스트로 변환했다.

다음으로, 정수 합을 구해주는데, 0이 나올 때까지 합을 구하고 구한 합은 새 리스트에 append했다. 그 뒤 0부터 다시 시작해 다음 0이 나올 때까지 합을 구했다. 마지막으로, 리스트 안에 음수가 몇 개 있는지 세서 리턴했다.

Refactoring

def countingValleys(n, s):
  lvl = 0
  vallery_cnt = 0
  for direction in s:
    if direction == 'U':
      lvl += 1
    else:
      if lvl == 0:
        vallery_cnt += 1
      lvl -= 1
  return vallery_cnt