HackerRank - 30 Days of Code - Day 2

Problem: Counting Valleys

An avid hiker keeps meticulous records of their hikes. During the last hike that took exactly steps steps, for every step it was noted if it was an uphill, U , or a downhill, D step. Hikes always 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 the sequence of up and down steps during a hike, find and print the number of valleys walked through.

Example

steps = 8 path = [DDUUUUDD]

The hiker first enters a valley 2 units deep. Then they climb out an up onto a mountain 2 units high. Finally, the hiker returns to sea level and ends the hike.

Function Description

Complete the countingValleys function in the editor below. countingValleys has the following parameter(s):

• int steps: the number of steps on the hike
• string path: a string describing the path

Returns

• int: the number of valleys traversed

Input Format

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

Constraints

• 2 <= steps <= 10^6
• path[i] ∈ {UD}

Sample Input

8 UDDDUDUU

Sample Output

1

Explanation

If we represent \_ as sea level, a step up as /, and a step down as \, the hike can be drawn as:

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1_/\      _
2   \    /
3    \/\/

The hiker enters and leaves one valley.

Solution

After thinking about the proplem, I came up with the following solution:

• Create a variable currentLevel to keep track of the current level.
• Create a variable valleys to keep track of the number of valleys.
• Loop through the path string.
• If the current character is U, increment currentLevel by 1.
• If the current character is D, decrement currentLevel by 1.
• If currentLevel is 0 and the current character is U, increment valleys by 1.
• Return valleys.

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1function countingValleys(steps, path) {
2  let currentLevel = 0;
3  let valleys = 0;
4
5  for (let i = 0; i < steps; i++) {
6    if (path[i] === 'D') {
7      currentLevel--;
8    } else if (path[i] === 'U') {
9      currentLevel++;
10    }
11
12    if (currentLevel === 0 && path[i] === 'U') {
13      valleys++;
14    }
15  }
16
17  return valleys;
18}

Time Complexity

O(n) where n is the number of steps.

Reference

• HackerRank - 30 Days of Code - Day 2