Absolute Permutation (from HackerRank)

-
This was a fun, moderate problem: https://www.hackerrank.com/challenges/absolute-permutation
It is asking you to find an absolute permutation for the given arrays. All you have to do is check whether you can find a suitable candidate for each position in the array. There are only two candidates: index+k and index-k. Keep track of the candidates already used, make sure to prioritize the ones in ascending order, and you should be good. It is in linear time, and given the upper bound of 10^5, this would have been a hint that you need to do it in linear time otherwise the TCs will timeout. Apparently it worked:


Code is below, cheers, Marcelo.

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Collections;
class Solution
{
    public static void Main(String[] args)
    {
        int t = Int32.Parse(Console.ReadLine());
        int[][] solution = new int[t][];

        for (int i = 0; i < t; i++)
        {
            string[] parts = Console.ReadLine().Split(' ');
            int n = Int32.Parse(parts[0]);
            int k = Int32.Parse(parts[1]);

            solution[i] = new int[n];

            Hashtable htUsed = new Hashtable();
            for (int j = 0; j < n; j++)
            {
                int index = j + 1;
                int candidate1 = index + k;
                int candidate2 = index - k;

                if (candidate1 > 0 &&
                    candidate1 <= n &&
                    !htUsed.ContainsKey(candidate1))
                {
                    if (candidate2 <= 0 ||
                        candidate2 > n ||
                        htUsed.ContainsKey(candidate2))
                    {
                        solution[i][j] = candidate1;
                        htUsed.Add(candidate1, true);
                    }
                    else
                    {
                        if (candidate1 < candidate2)
                        {
                            solution[i][j] = candidate1;
                            htUsed.Add(candidate1, true);
                        }
                        else
                        {
                            solution[i][j] = candidate2;
                            htUsed.Add(candidate2, true);
                        }
                    }
                }
                else if (candidate2 > 0 &&
                         candidate2 <= n &&
                         !htUsed.ContainsKey(candidate2))
                {
                    if (candidate1 <= 0 ||
                        candidate1 > n ||
                        htUsed.ContainsKey(candidate1))
                    {
                        solution[i][j] = candidate2;
                        htUsed.Add(candidate2, true);
                    }
                    else
                    {
                        if (candidate2 < candidate1)
                        {
                            solution[i][j] = candidate2;
                            htUsed.Add(candidate2, true);
                        }
                        else
                        {
                            solution[i][j] = candidate1;
                            htUsed.Add(candidate1, true);
                        }
                    }
                }
                else
                {
                    solution[i] = new int[1];
                    solution[i][0] = -1;
                    break;
                }
            }
        }

        for (int i = 0; i < t; i++)
        {
            for (int j = 0; j < solution[i].Length; j++)
            {
                Console.Write("{0} ", solution[i][j]);
            }
            Console.WriteLine();
        }
    }
}

Comments

  1. TarasDecember 6, 2017 at 11:00 PM

    I'm not sure why this problem was marked as medium, since it required only a very basic math and coding, but it's still a nice little problem. My solution is pretty much the same, except that I didn't want to accumulate as much memory (storing the entire grid can be pretty expensive in terms of memory) and used a bitset instead of hashtable for checking used values, since most of the time it's going to be full anyways, so hashtable only adds performance and memory overhead :)

    #include
    #include
    #include
    using namespace std;

    int value_at(int i, int k, const vector used) {
    // abs(x - i) = k:
    // 1) x - i = k => x = k + i
    // 2) x - i = -k => x = -k + i
    int case_2 = -k + i;
    // case 2 is preferable (smaller) as long as it's still unused
    if (case_2 > 0 && !used[case_2]) return case_2;
    return k + i; // case 1
    }

    vector permutation_for(int K, int N) {
    vector permutation(N);
    vector used(N+1, false);
    for (int i = 1; i <= N; ++i) {
    int value = value_at(i, K, used);
    if (value > N || used[value]) return {};
    permutation[i-1] = value;
    used[value] = true;
    }
    return permutation;
    }

    int main() {
    int T; cin >> T;
    for (int t = 0; t < T; ++t) {
    int N, K; cin >> N >> K;
    vector permutation = permutation_for(K, N);
    if (permutation.empty()) {
    cout << "-1";
    } else {
    copy(permutation.cbegin(), permutation.cend(), ostream_iterator(cout, " "));
    }
    cout << endl;
    }
    return 0;
    }

    Cheers,
    Taras

    ReplyDelete

Post a Comment

Popular posts from this blog

Advent of Code - Day 6, 2024: BFS and FSM

-
Image
To solve this Advent of Code (parts 1 and 2), I used Breadth-First Search (BFS) to keep moving the guard as well as Finite State Machine (FSM) to control the directions. This solves part 1 easily. Part 2 can be solved using the same technique, takes a little longer since you need to try every empty cell, but it eventually does the trick (takes a min or so). Code is down below, cheers, ACC. Day 6 - Advent of Code 2024 using System.IO; using System.Collections; using System; using System.Text; using System.Text.RegularExpressions; using System.ComponentModel.DataAnnotations; Process2(); void Process() { string fileName = "input.txt"; FileInfo fileInfo = new FileInfo(fileName); StreamReader sr = fileInfo.OpenText(); List map = new List (); int startRow = 0; int startCol = 0; while (!sr.EndOfStream) { string line = sr.ReadLine().Trim(); map.Add(new StringBuilder(line)); int indexStart = line.IndexOf('^'); ...
Read more

Golang vs. C#: performance characteristics (simple case study)

-
Image
This is an interesting example to study the performance characteristics of Golang compared to C# for a very particular case (not generalized though).  The same code was written in C# and Golang (literally translated from one to the other). Below you'll be able to see the source code for both. The problem is a simple post-order DFS in a tree-like graph. The code in C# times out (TLE = Time Limited Exceeded). The code in Golang not only works but beats 100% of the other Golang submissions. Take a look the image below: Some potential reasons for the performance gains from Go in this particular example are: 1. Language and Runtime Differences:    Go is compiled directly to machine code, while C# typically runs on a virtual machine (CLR).    Go has a simpler runtime with less overhead compared to the .NET runtime.    Go's garbage collector is designed for low latency, which can lead to better performance in some scenarios. 2. Data Structures:    G...
Read more

Advent of Code - Day 7, 2024: Backtracking and Eval

-
Image
Problem today requires backtracking and eval (evaluating a mathematical expression in the format of a string). Glad that it used a left-2-right evaluation, makes for an easy parsing and not overly complicated evaluation function. Code is down below for parts 1 and 2. Cheers, ACC. Day 7 - Advent of Code 2024 using System.IO; using System.Collections; using System; using System.Text; using System.Text.RegularExpressions; using System.ComponentModel.DataAnnotations; Process2(); void Process() { string fileName = "input.txt"; long retVal = 0; FileInfo fileInfo = new FileInfo(fileName); StreamReader sr = fileInfo.OpenText(); while (!sr.EndOfStream) { string line = sr.ReadLine().Trim(); string[] parts1 = line.Split(':'); long target = Int64.Parse(parts1[0]); string[] numbers = parts1[1].Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries); if (TryAll1(numbers, target, "", 0)) r...
Read more