cs: add «2661. First Completely Painted Row or Column»

URL:	https://leetcode.com/problems/first-completely-painted-row-or-column/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/first-completely-painted-row-or-column.cs

@@ -0,0 +1,36 @@
+public class Solution {
+    private bool InBounds(int[][] mat, int x, int y)
+        => y >= 0 && y < mat.Length && x >= 0 && x < mat[y].Length;
+
+    private int Check(int[][] mat, Dictionary<int, int> indices, (int, int) start, (int, int) d) {
+        var lastToPaint = int.MinValue;
+
+        var (dx, dy) = d;
+        for (var (x, y) = start; InBounds(mat, x, y); x += dx, y += dy) {
+            lastToPaint = Math.Max(lastToPaint, indices[mat[y][x]]);
+        }
+
+        return lastToPaint;
+    }
+
+    public int FirstCompleteIndex(int[] arr, int[][] mat) {
+        var indices = new Dictionary<int, int>();
+        for (var i = 0; i < arr.Length; ++i) {
+            indices[arr[i]] = i;
+        }
+
+        var first = int.MaxValue;
+
+        // Check through rows
+        for (var y = 0; y < mat.Length; ++y) {
+            first = Math.Min(first, Check(mat, indices, (0, y), (1, 0)));
+        }
+
+        // Check through columns
+        for (var x = 0; x < mat[0].Length; ++x) {
+            first = Math.Min(first, Check(mat, indices, (x, 0), (0, 1)));
+        }
+
+        return first;
+    }
+}