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Counting Valleys - Coding Challenge

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 steps. For every step he took, he noted if it was an uphill, , or a downhill, step. Gary's hikes start and end at sea level and each step up or down represents a 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 , he first enters a valley units deep. Then he climbs out an up onto a mountain 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 , the number of steps in Gary's hike.
The second line contains a single string , of characters that describe his path.


Output Format

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

Sample Input


Sample Output



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.


This is coding challenge labelled easy on Hacker rank but it was not so easy for me. It took me about 45 minutes to solve, thus easy is relative term.

In order to solve the problem we have to take into consideration what is asked of us, that is we are only interested in counting the number of valleys. From the sample above if you convert the U to +1 and D to -1, the sum will be 0, and of the U and D, we end the stream at U. So you can conclude from that pattern that a valley is constituted by U & D summing up to 0 and the last char in the string being equal to U. We can represent this algorithm by code as follows:

function countingValleys(n, s) {
    let level = 0;
    let valley = 0;
    s.split('').forEach(item => {
        if (item === 'U') {
        } else{
        if (item === 'U' && level === 0) {

    return valley;

Full Solution

Below is the full solution which you can run in node js.

'use strict';

const fs = require('fs');


let inputString = '';
let currentLine = 0;

process.stdin.on('data', inputStdin => {
    inputString += inputStdin;

process.stdin.on('end', _ => {
    inputString = inputString.replace(/\s*$/, '')
        .map(str => str.replace(/\s*$/, ''));


function readLine() {
    return inputString[currentLine++];

// Complete the countingValleys function below.
function countingValleys(n, s) {
    let lvl = 0;
    let v = 0;
    s.split('').forEach(item => {
        if (item === 'U') {
        } else{
        if (item === 'U' && lvl === 0) {

    return v;

function main() {
    const ws = fs.createWriteStream(process.env.OUTPUT_PATH);

    const n = parseInt(readLine(), 10);

    const s = readLine();

    let result = countingValleys(n, s);

    ws.write(result + "\n");


Thank you for reading my first ever article.

My name is Letlhogonolo Theodore Obonye
I'm a JavaScript developer

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