Develop Reference

HackerRank Climbing the Leaderboard

This question has to do with this challenge on HackerRank. It seems to be failing some cases, but I'm not clear what's wrong with the algorithm (many people seem to have problem with timeouts, that's not an issue here, everything runs plenty fast, and all the cases that are visible to me pass, so I don't have a specific case that's failing).
The essential outline of how the algorithm works is as follows:
First be sure that Alice isn't already winning over the existing highest score (degenerate case), if she is just tell the world she's #1 from start to finish. Otherwise, at least one score on the leaderboard beats Alice's first try.
Start by walking down the scores list from the highest until we find a place where Alice fits in and record the scores that beat Alice's initial score along the way.
If we reach the end of the scores list before finding a place for Alice's bottom score, pretend there is a score at the bottom of the list which matches Alice's first score (this is just convenient for the main loop and reduces the problem to one where Alice's first score is on the list somewhere)
At this point we have a (sorted) array of scores with their associated ranks, rankAry[r - 1] is the minimum score needed for Alice to attain rank r as of the end of the if clause following the first while loop.
From there, the main algorithm takes over where we walk through Alice's scores and note her rank as we go by comparing against the benchmarks from the scores array that we setup as rankAry earlier. curRank is our candidate rank at each stage which we've definitely achieved by the time this loop starts (by construction).
If we're at rank 1 we will be forever more, so just populate the current rank as 1 and move on.
If we're currently tied with or beating the current benchmark and that's not the end of the line, keep peeking at the next one and if we're also beating that next one, decrease the current benchmark location and iterate
Once this terminates, we've found the one we're going to supplant and we cannot supplant anything further, so assign this rank to this score and repeat until done
As far as I can tell this handles all cases correctly, even if Alice has repeated values or increases between the benchmarks from scores, we should stay at the same rank until we hit the new benchmarks, but the site feedback indicates there must be a bug somewhere.
All the other approaches I've been able to find seem to be some variation on doing a binary search to find the score each time, but I prefer not having to constantly search each time and just use the auxiliary space, so I'm a little stumped on what could be off.
static int[] climbingLeaderboard(int[] scores, int[] alice) {
int[] res = new int[alice.Length];
if (scores.Length == 0 || alice[0] >= scores[0]) { //degenerate cases
for (int i = 0; i < alice.Length; ++i) {
res[i] = 1;
}
return res;
}
int[] rankAry = new int[scores.Length + 1];
rankAry[0] = scores[0]; //top score rank
int curPos = 1; //start at the front and move down
int curRank = 1; //initialize
//initialize from the front. This way we can figure out ranks as we go
while (curPos < scores.Length && scores[curPos] > alice[0]) {
if (scores[curPos] < scores[curPos-1]) {
rankAry[curRank] = scores[curPos]; //update the rank break point
curRank++; //moved down in rank
}
curPos++; //move down the array
}
if (curPos == scores.Length) { //smallest score still bigger than Alice's first
rankAry[curRank] = alice[0]; //pretend there was a virtual value at the end
curRank++; //give rank Alice will have for first score when we get there
}
for (int i = 0; i < alice.Length; ++i) {
if (curRank == 1) { //if we're at the top, we're going to stay there
res[i] = 1;
continue;
}
//Non-degenerate cases
while (alice[i] >= rankAry[curRank - 1]) {
if (curRank == 1 || alice[i] < rankAry[curRank - 2]) {
break;
}
curRank--;
}
res[i] = curRank;
}
return res;
}

You have a couple of bugs in your algorithm.
Wrong mapping
Your rankAry must map a rank (your index) to a score. However, with this line rankAry[0] = scores[0];, the highest score is mapped to 0, but the highest possible rank is 1 and not 0. So, change that to:
rankAry[1] = scores[0];
Wrong initial rank
For some reason, your curRank is set to 1 as below:
int curRank = 1; //initialize
However, it's wrong since your alice[0] is less than scores[0] because of the following block running at the beginning of your method:
if (scores.Length == 0 || alice[0] >= scores[0]) { //degenerate cases
for (int i = 0; i < alice.Length; ++i) {
res[i] = 1;
}
return res;
}
So, at best your curRank is 2. Hence, change it to:
int curRank = 2;
Then, you can also remove curRank++ as your curRank has a correct initial value from:
if (curPos == scores.Length) { //smallest score still bigger than Alice's first
rankAry[curRank] = alice[0]; //pretend there was a virtual value at the end
curRank++; // it's not longer needed so remove it
}
Improve "Non-degenerate cases" handling
Your break condition should consider rankAry at curRank - 1 and not curRank - 2 as it's enough to check the adjacent rank value. Also, a value at curRank - 2 will produce wrong results for some input but I won't explain for which cases specifically - I'll leave it up to you to find out.
Fixed Code
So, I fixed your method according to my comment above and it passed it all the tests. Here it is.
static int[] climbingLeaderboard(int[] scores, int[] alice) {
int[] res = new int[alice.Length];
if (scores.Length == 0 || alice[0] >= scores[0]) { //degenerate cases
for (int i = 0; i < alice.Length; ++i) {
res[i] = 1;
}
return res;
}
int[] rankAry = new int[scores.Length + 1];
rankAry[1] = scores[0]; //top score rank
int curPos = 1; //start at the front and move down
int curRank = 2; //initialize
//initialize from the front. This way we can figure out ranks as we go
while (curPos < scores.Length && scores[curPos] > alice[0]) {
if (scores[curPos] < scores[curPos-1]) {
rankAry[curRank] = scores[curPos]; //update the rank break point
curRank++; //moved down in rank
}
curPos++; //move down the array
}
if (curPos == scores.Length) { //smallest score still bigger than Alice's first
rankAry[curRank] = alice[0]; //pretend there was a virtual value at the end
}
for (int i = 0; i < alice.Length; ++i) {
if (curRank == 1) { //if we're at the top, we're going to stay there
res[i] = 1;
continue;
}
//Non-degenerate cases
while (alice[i] >= rankAry[curRank - 1]) {
if (curRank == 1 || alice[i] < rankAry[curRank - 1]) {
break;
}
curRank--;
}
res[i] = curRank;
}
return res;
}

Find lowest period of a string (given array of string) to create a substring and return an array of integers

So I was looking through interview questions that unfortunately didn't have any solutions, and one of them seemed interesting (mostly because I had no idea how to do it). However now I have hit a roadblock and am not sure how to proceed. The objective of the program is to take any amount of strings and spit out the periods of the lowest repeated characters to form the substring in an integer array.
Ex: "abbabbabba","abcdabcddabcdabc","aeiouuaeioouaei" would be 5, 8, 5.
I kind of looked at it as doing Indian runs. The first character runs to the front, if it equals the next character than continue running, etc.
This is the code I have compiled and I tried to take a look at the algorithm for finding periods as well (http://monge.univ-mlv.fr/~mac/REC/text-algorithms.pdf#page=345). I just can't seem to pinpoint the problem in the code
namespace Interview36{
public class PeriodicStrings
{
public static int[] process(string[] input)
{
if (input == null || input.Length == 0 || input[0].Length == 0)
{
Console.WriteLine("Result is null");
}
int length = 0;
int[] list = new int[input.Length];
for (int i = 0; i < input.Length; i++) {
while (i < input[i].Length)
{
if (input[i] == input[length])
{
length++;
list[i] = length;
i++;
}
else
{
if (length != 0)
{
length = list[length - 1];
}
else
{
list[i] = 0;
i++;
}
}
}
}
return list;
}
}
}
My best guess is that my logic is WAY off, or the loop isn't running through properly and I'm just not catching it in debug. Thank you in advance for any insight!

How to optimize a loops going through arrays?

I've been going though www.testdome.com to test my skills and opened a list of public questions. One of the practice questions was:
Implement function CountNumbers that accepts a sorted array of
integers and counts the number of array elements that are less than
the parameter lessThan.
For example, SortedSearch.CountNumbers(new int[] { 1, 3, 5, 7 }, 4)
should return 2 because there are two array elements less than 4.
And my answer was:
using System;
public class SortedSearch
{
public static int CountNumbers(int[] sortedArray, int lessThan)
{
int count = 0;
int l = sortedArray.Length;
for (int i = 0; i < l; i++) {
if (sortedArray [i] < lessThan)
count++;
}
return count;
}
public static void Main(string[] args)
{
Console.WriteLine(SortedSearch.CountNumbers(new int[] { 1, 3, 5, 7 }, 4));
}
}
It seems that I've failed on two counts:
Performance test when sortedArray contains lessThan: Time limit exceeded
and
Performance test when sortedArray doesn't contain lessThan: Time limit exceeded
To be honest I'm not sure what to optimize there? Maybe I'm using a wrong method and there is a similar way to speed up the calculation?
If someone could point out my mistake or explain what I'm going wrong, I'd really appreciate it!

Because the array is sorted, you can stop counting as soon as you reach or exceed the lessThan parameter.
else break would probably do it.

Does it have to be really a loop? You could do Lambda exp for that
public static int CountNumbers(int[] sortedArray, int lessThan)
{
return sortedArray.ToList().Where(x=>x < lessThan).Count();
}

Harold's answer and approach is spot on.
Find below another code sample in case you're practicing for technical interviews. It handles cases when the array is null or empty, when lessThan is presented in the array (including duplicates), etc.
private static int CountNumbers(int[] sortedArray, int lessThan)
{
if (sortedArray == null)
{
throw new ArgumentNullException("Sorted array cannot be null.");
}
if (sortedArray.Length == 0)
{
throw new ArgumentException("Sorted array cannot be empty.");
}
int start = 0;
int end = sortedArray.Length;
int middle = int.MinValue;
while (start < end)
{
middle = (start + end) / 2;
if (sortedArray[middle] == lessThan)
{
break; // Found the "lessThan" number in the array, we can stop and move left
}
else if (sortedArray[middle] < lessThan)
{
start = middle + 1;
}
else
{
end = middle - 1;
}
}
// Adjust the middle pointer based on the "current" and "lessThan" numbers in the sorted array
while (middle >= 0 && sortedArray[middle] >= lessThan)
{
middle--;
}
// +1 because middle is calculated through 0-based (e.g. start)
return middle + 1;
}

Current code to generate array filled with unique ints crashes

(this is a library)
The function GetUniqueInt is being called with (5, 5) as the variables.
Currently the code will stall unity to a complete halt, or crash my PC with a memory overflow error.
Does anyone have any ideas as to how I could prevent it from crashing or what is making it crash?
using UnityEngine;
namespace MajorSolution
{
public static class MajorMath
{
public static int[] GetUniqueInt(int intCount, int intLength)
{
int[] returnValue = new int[intCount];
int[] temp = new int[intLength];
for (int a = 0; a < intCount; a++)
{
string create = new string("create".ToCharArray());
switch (create)
{
case "create":
returnValue[a] = GetRandomInt(intCount);
goto case "check";
case "check":
bool alreadyTaken = false;
for (int c = 0; c < returnValue.Length - 1; c++)
{
if (returnValue[a] == returnValue[c])
{
// Already Taken!
alreadyTaken = true;
}
}
if (!alreadyTaken)
{
break;
}
else
{
goto case "create";
}
}
}
Debug.Log(returnValue);
return returnValue;
}
public static int GetRandomInt(int intCount)
{
int[] storage = new int[intCount];
int returnValue = 0;
for (int i = 0; i < intCount; i++)
{
storage[i] = (Mathf.FloorToInt(Random.Range(0, 9)) * (int)Mathf.Pow(10,i));
returnValue += storage[i];
}
return returnValue;
}
}
}

Edit I just realized I did not exactly answer the question of why it is bringing the PC to a halt because you have an infinite loop in the code.
The problem is occurring in the following lines of code, notice what is happening.
case "create":
returnValue[a] = GetRandomInt(intCount);
goto case "check";
In the above block of code you are generating a number and putting it in the returnValue array. Now you jump into your "check" block
case "check":
bool alreadyTaken = false;
for (int c = 0; c < returnValue.Length - 1; c++)
{
if (returnValue[a] == returnValue[c])
{
// Already Taken!
alreadyTaken = true;
}
}
In this block of code you are looping over the entire returnValue array including the value you just inserted in it. Basically you are looping over the array asking if a value that you just put in the array is in the array.
Without knowing exactly what you are trying to do with these methods, I will just make a simple fix suggestion with some minor cleanup
public static int[] GetUniqueInt(int count, int length)
{
var returnValue = new int[count];
var values = new HashSet<int>(); // Used to track what numbers we have generated
for (int i = 0; i < count; ++i)
{
// Generate the number and check to be sure we haven't seen it yet
var number = GetRandomInt(length);
while(values.Contains(number)) // This checks if the number we just generated exists in the HashSet of seen numbers
{
// We get here if the HashSet contains the number. If we have
// seen the number then we need to generate a different one
number = GetRandomInt(length);
}
// When we reach this point, it means that we have generated a new unique number
// Add the number to the return array and also add it to the list of seen numbers
returnValue[a] = number;
values.Add(number); // Adds the number to the HashSet
}
Debug.Log(returnValue);
return returnValue;
}
I did end up removing the using of intLength but from your posted code it was only used to declare a temp array which itself was never used. Based on that, I just removed it entirely.
Based on your comment I updated the fix to use intLength. I made one other minor change. I removed int from the variable names of count and length. Hungarian notation is a lot less common in C# code. Personally, I feel like the code is cleaner and easier to read without the Hungarian notation. The key is to use good variable names that express the intent or make it easier to follow. In this case count is the count (read total number) of numbers you want returned and length is the length of the number. You could consider maybe even renaming that to numberOfDigits to make it clearer that the idea is you are going to create a random number with that number of digits in it.

Generating Schröder Paths

I want to generate schröder paths from (0, 0) to (2n, 0) with
no peaks, i.e., no up step followed immediately by a down step.
Some examples are for n=3 : shröder paths.
/ is coded as U, -- is coded as R and \ is coded as D. Here is my code to generate these paths :
public static void addParen(List<String> list, int upstock,int rightstock,int
downstock,bool B, char[] str, int count,int total,int n)
{
if (total == n && downstock == 0)
{
String s = copyvalueof(str);
list.Add(s);
}
if (total > n || (total==n && downstock>0) )
return;
else
{
if (upstock > 0 && total<n)
{
str[count] = 'U';
addParen(list, upstock - 1,rightstock, downstock+1,B=true, str, count + 1,total+1,n);
}
if (downstock > 0 && total<n && B==false)
{
str[count] = 'D';
addParen(list, upstock,rightstock, downstock - 1,B=false, str, count + 1,total+1,n);
}
if (rightstock > 0 && total < n)
{
str[count] = 'R';
addParen(list, upstock, rightstock-1, downstock, B = false, str, count + 1, total + 2,n);
}
}
}
public static List<String> generatePaths(int count)
{
char[] str = new char[count * 2];
bool B = false;
List<String> list = new List<String>();
addParen(list, count-1, count, 0,B,str, 0, 0,count*2);
return list;
}
The total is 2n. I start with n-1 ups n rights and zero downs.Since there is no Up yet my bool B is false ( if there comes an up then down can not come after it,so to prevent this I put B=true which prevents it.) If an up comes, then there should be a corresponding down, and total should be incremented by one. If right comes then, total should be incremented by 2. My algorithm in general works like that but I couldn't get a correct result with this implementation.

The original solution didn't adapt to OP's needs, as it was too complicated to port to javascript and the intention was to show better practices solving these kind of problems more than actually solving easily this one in particular.
But in the spirit of using immutable types to solve path algorithms, we can still do so in a much simpler fashion: we'll use string.
Ok as always, lets build up our infrastructure: tools that will make our life easier:
private const char Up = 'U';
private const char Down = 'D';
private const char Horizontal = 'R';
private static readonly char[] upOrHorizontal = new[] { Up, Horizontal };
private static readonly char[] downOrHorizontal = new[] { Down, Horizontal };
private static readonly char[] all = new[] { Up, Horizontal, Down };
And a handy little helper method:
private static IList<char> GetAllPossibleDirectionsFrom(string path)
{
if (path.Length == 0)
return upOrHorizontal;
switch (path.Last())
{
case Up: return upOrHorizontal;
case Down: return downOrHorizontal;
case Horizontal: return all;
default:
Debug.Assert(false);
throw new NotSupportedException();
}
}
Remember, break down your problems to smaller problems. All difficult problems can be solved solving smaller easier problems. This helper method is hard to get wrong; thats good, its hard to write a bug inside easy short methods.
And now, we solve the bigger problem. We'll not use iterator blocks so porting is easier. We'll concede here using a mutable list to keep track of all the valid paths we find.
Our recursive solution is the following:
private static void getPaths(IList<string> allPaths,
string currentPath,
int height,
int maxLength,
int maxHeight)
{
if (currentPath.Length == maxLength)
{
if (height == 0)
{
allPaths.Add(currentPath);
}
}
else
{
foreach (var d in GetAllPossibleDirectionsFrom(currentPath))
{
int newHeight;
switch (d)
{
case Up:
newHeight = height + 1;
break;
case Down:
newHeight = height - 1;
break;
case Horizontal:
newHeight = height;
break;
default:
Debug.Assert(false);
throw new NotSupportedException();
}
if (newHeight < 0 /*illegal path*/ ||
newHeight >
maxLength - (currentPath.Length + 1)) /*can not possibly
end with zero height*/
continue;
getPaths(allPaths,
currentPath + d.ToString(),
newHeight,
maxLength,
maxHeight);
}
}
}
Not much to say, its pretty self explanatory. We could cut back some on the arguments; height is not strictly necessary, we could count ups and downs in the current path and figure out the height we are currently at, but that seems wasteful. maxLength could also, and probably should be removed, we have enough information with maxHeight.
Now we just need a method to kick this off:
public static IList<string> GetSchroderPathsWithoutPeaks(int n)
{
var allPaths = new List<string>();
getPaths(allPaths, "", 0, 2 * n, n);
return allPaths;
}
And we're set! If we take this out for a test drive:
var paths = GetSchroderPathsWithoutPeaks(2);
Console.WriteLine(string.Join(Environment.NewLine, paths));
We get the expected results:
URRD
URDR
RURD
RRRR
As to why your solution doesn't work? Well, just the fact that you can't figure it out says it all about how unecessary complicated your current solution is starting to look. When that happens, its normally a good idea to take a step back, rethink your approach, write down a clear specification of what your program should be doing step by step and start over.