[Hacker Rank] Interview Preparation Kit: Greedy Algorithms: Luck Balance. Solved ✓.

Author: Gonzalo Diaz

algorithm-exercises-csharp-test/src/hackerrank/interview_preparation_kit/greedy_algorithms/LuckBalance.Test.cs

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+namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit;
+
+[TestClass]
+public class LuckBalanceTest
+{
+  public class LuckBalanceTestCase
+  {
+    public string title = "";
+    public int k;
+    public List<List<int>> contests = [];
+    public int expected;
+  }
+
+  private static readonly LuckBalanceTestCase[] tests = [
+    new()
+    {
+      title = "Sample Test case 0",
+      k = 3,
+      contests = [[5, 1], [2, 1], [1, 1], [8, 1], [10, 0], [5, 0]],
+      expected = 29
+    },
+    new()
+    {
+      title = "Sample Test case 1",
+      k = 5,
+      contests = [[13, 1], [10, 1], [9, 1], [8, 1], [13, 1], [12, 1], [18, 1], [13, 1]],
+      expected = 42
+    },
+    new()
+    {
+      title = "Sample Test case 2",
+      k = 2,
+      contests = [[5, 1], [4, 0], [6, 1], [2, 1], [8, 0]],
+      expected = 21
+    }
+  ];
+
+  [TestMethod]
+  public void testLuckBalance()
+  {
+    int result;
+
+    foreach (LuckBalanceTestCase test in tests)
+    {
+      result = LuckBalance.luckBalance(test.k, test.contests);
+      Assert.AreEqual(
+        test.expected,
+        result,
+        String.Format(
+          "testLuckBalance({0}, {1}) => must be: {2}",
+          test.k,
+          test.contests.ToString(),
+          test.expected
+        )
+      );
+    }
+  }
+}

algorithm-exercises-csharp/src/hackerrank/interview_preparation_kit/greedy_algorithms/LuckBalance.cs

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+// @link Problem definition [[docs/hackerrank/interview_preparation_kit/greedy_algorithms/luck-balance.md]]
+
+namespace algorithm_exercises_csharp.hackerrank.interview_preparation_kit;
+
+using System.Diagnostics.CodeAnalysis;
+
+public class Competition(int _luck, int _important)
+{
+  public int luck => _luck;
+  public int important => _important;
+}
+
+public class LuckBalance
+{
+  [ExcludeFromCodeCoverage]
+  protected LuckBalance() { }
+
+  public static int luckBalance(int k, List<List<int>> contests)
+  {
+    List<Competition> important_competitions = [];
+    List<Competition> nonimportant_competitions = [];
+
+    foreach (var x in contests)
+    {
+      var luck = x[0];
+      var important = x[1];
+
+      if (important == 1)
+      {
+        important_competitions.Add(new Competition(luck, important));
+      }
+      else
+      {
+        nonimportant_competitions.Add(new Competition(luck, important));
+      }
+    }
+
+    important_competitions = [.. important_competitions
+      .OrderByDescending(s => s.important)
+      .ThenByDescending(s => s.luck)];
+
+    int total = 0;
+    int size = important_competitions.Count;
+
+    for (int i = 0; i < Math.Min(k, size); i++)
+    {
+      total += important_competitions[i].luck;
+    }
+
+    for (int i = Math.Min(k, size); i < size; i++)
+    {
+      total -= important_competitions[i].luck;
+    }
+
+    foreach (Competition x in nonimportant_competitions)
+    {
+      total += x.luck;
+    }
+
+    return total;
+  }
+}

docs/hackerrank/interview_preparation_kit/greedy_algorithms/luck-balance.md

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+# [Greedy Algorithms: Luck Balance](https://www.hackerrank.com/challenges/luck-balance)
+
+- Difficulty:  `#easy`
+- Category: `#ProblemSolvingBasic`
+
+Lena is preparing for an important coding competition that is preceded
+by a number of sequential preliminary contests.
+Initially, her luck balance is 0.
+She believes in "saving luck", and wants to check her theory.
+Each contest is described by two integers, `L[i]` and `T[i]`:
+
+- `L[i]` is the amount of luck associated with a contest.
+If Lena wins the contest, her luck balance will decrease by `L[i]`;
+if she loses it, her luck balance will increase by `L[i]`.
+
+- `T[i]` denotes the contest's importance rating.
+It's equal to `1` if the contest is important, and it's equal to `0` if it's unimportant.
+
+If Lena loses no more than `k` important contests, what is the maximum amount
+of luck she can have after competing in all the preliminary contests?
+This value may be negative.
+
+## Example
+
+```text
+Contest     L[i]    T[i]
+1           5       1
+2           1       1
+3           4       0
+```
+
+If Lena loses all of the contests, her will be `5 + 1 +4 = 10`.
+Since she is allowed to lose  important contests,
+and there are only `2` important contests,
+she can lose all three contests to maximize her luck at `10`.
+
+If `k = 1`, she has to win at least  of the  important contests.
+She would choose to win the lowest value important contest worth `1`.
+Her final luck will be `5 + 4 - 1 = 8`.
+
+## Function Description
+
+Complete the luckBalance function in the editor below.
+
+luckBalance has the following parameter(s):
+
+- `int k`: the number of important contests Lena can lose
+- `int contests[n][2]`: a 2D array of integers where each `contests[i]`
+contains two integers that represent the luck balance and importance of the  contest
+
+## Returns
+
+- `int`: the maximum luck balance achievable
+
+## Input Format
+
+The first line contains two space-separated integers  `n` and `k`,
+the number of preliminary contests and the maximum number
+of important contests Lena can lose.
+
+Each of the next  lines contains two space-separated integers,
+`L[i]` and `T[i]`, the contest's luck balance and its importance rating.
+
+## Constraints
+
+- $ 1 \leq n \leq 100 $
+- $ 0 \leq k \leq N $
+- $ 1 \leq L[i] \leq 10^4 $
+- $ T[i] \isin \{0,1\} $
+
+## Sample Input
+
+```text
+STDIN       Function
+-----       --------
+6 3         n = 6, k = 3
+5 1         contests = [[5, 1], [2, 1], [1, 1], [8, 1], [10, 0], [5, 0]]
+2 1
+1 1
+8 1
+10 0
+5 0
+```
+
+## Sample Output
+
+```text
+29
+```
+
+## Explanation
+
+There are `n = 6` contests. Of these contests, `4` are important
+and she cannot lose more than  of them.
+Lena maximizes her luck if she wins the $ 3^{rd} $ important contest
+(where `L[i] = 1`) and loses all of the other five contests for a total
+luck balance of `5 + 2 + 8 + 10 + 5 - 1 = 29`.