[#498] feat(FindDiagonalOrder): add C# solution

202508/498 FindDiagonalOrder/C#/C#.sln

@@ -0,0 +1,24 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.5.2.0
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "FindDiagonalOrder", "FindDiagonalOrder.csproj", "{B3151728-C057-4305-DD4C-9A2AFBFC189A}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{B3151728-C057-4305-DD4C-9A2AFBFC189A}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{B3151728-C057-4305-DD4C-9A2AFBFC189A}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{B3151728-C057-4305-DD4C-9A2AFBFC189A}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{B3151728-C057-4305-DD4C-9A2AFBFC189A}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {DB56539A-6F32-4D28-B91F-BB1DCF46F4D0}
+	EndGlobalSection
+EndGlobal

202508/498 FindDiagonalOrder/C#/Program.cs

@@ -1,5 +1,61 @@
 public class Solution {
-    public int[] FindDiagonalOrder(int[][] mat) {
-        
+    public int[] FindDiagonalOrder(int[][] mat)
+    {
+        if (mat == null || mat.Length == 0 || mat[0].Length == 0)
+        {
+            return new int[0];
+        }
+
+        var m = mat.Length;
+        var n = mat[0].Length;
+
+        var diagonals = new Dictionary<int, List<int>>();
+
+        for (var i = 0; i < m; i++)
+        {
+            for (var j = 0; j < n; j++)
+            {
+                int diagonalIndex = i + j;
+                if (!diagonals.ContainsKey(diagonalIndex))
+                {
+                    diagonals[diagonalIndex] = new List<int>();
+                }
+                diagonals[diagonalIndex].Add(mat[i][j]);
+            }
+        }
+
+        var result = new List<int>();
+        foreach (var key in diagonals.Keys)
+        {
+            var diagonal = diagonals[key];
+            if (key % 2 == 0)
+            {
+                result.AddRange(diagonal);
+            }
+            else
+            {
+                diagonal.Reverse();
+                result.AddRange(diagonal);
+            }
+        }
+
+        return result.ToArray();
+    }
+}
+
+class Program
+{
+    static void Main()
+    {
+        var Solution = new Solution();
+
+        int[][] mat1 = new int[][]{
+            new int[]{1,2,3},
+            new int[]{4,5,6},
+            new int[]{7,8,9}
+        };
+
+        Console.WriteLine(string.Join(", ", Solution.FindDiagonalOrder(mat1)));
+
     }
 }