[csharp/matrix] Basic csharp mentoring notes for the matrix problem

tracks/csharp/exercises/matrix/mentoring.md

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+### Reasonable solutions: 
+
+Solution without Linq:
+```csharp
+using System.Collections.Generic;
+
+public class Matrix
+{
+  private readonly int[][] matrix;
+
+  public Matrix(string input)
+  {
+    var output = new List<int[]>();
+
+    foreach(var rowString in input.Split("\n")) {
+      var row = new List<int>();
+      foreach(var v in rowString.Split(" ")) {
+        row.Add(int.Parse(v));
+      }
+      output.Add(row.ToArray());
+    }
+
+    matrix = output.ToArray();
+  }
+
+  public IEnumerable<int> Row(int row)
+    => matrix[row-1];
+
+  public IEnumerable<int> Column(int col)
+  {
+    foreach(var row in matrix) { 
+      yield return row[col-1]; 
+    }
+  }
+}
+```
+
+Solution with Linq:
+```csharp
+using System.Linq;
+using System.Collections.Generic;
+
+public class Matrix
+{
+    private readonly int[][] matrix;
+
+    public Matrix(string input)
+    {
+        matrix = input
+          .Split("\n")
+          .Select(r => r.Split(" ").Select(x => int.Parse(x)).ToArray())
+          .ToArray();
+    }
+
+    public IEnumerable<int> Row(int row)
+      => matrix[row-1];
+
+    public IEnumerable<int> Column(int col)
+      => matrix.Select(x => x[col-1]);
+}
+```
+
+### Common suggestions:
+*Good suggestions specific to this exercise. Good lessons that emerge from it.*
+
+- While it is possible to solve the problem using multidimensional arrays the jagged array is easier to work with.
+This is because it is possible to return a single row simply as `matrix[r]`.  
+
+### Talking points:
+*Questions to challenge more advance learners with.*
+
+- The differences between multidimensional and jagged arrays in general and in C#.
+In this case the data is dense so there are no memory efficiencies to talk about. 
+However, the choice influeces the type of code we can write. 
+- There are two problems: parsing of input and responding to queries. 
+Both are relatively simple and there is no need for input checking. 
+- The problem can be solved with iteration using loops and using Linq. What's the difference? 
+Both the parsing and access to column data change. 
+The change in parsing is more obvious between very imperative use of two loops vs more declarative expression using linq.