cs: add «79. Word Search»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/word-search.cs

@@ -0,0 +1,55 @@
+public class Solution {
+    private struct Solver {
+        char[][] Board { get; init; }
+        int Height { get => Board.Length; }
+        int Width { get => Board[0].Length; }
+
+        string Word { get; init; }
+
+        public Solver(char[][] board, string word) {
+            Board = board;
+            Word = word;
+        }
+
+        private bool Has(int y, int x)
+            => y >= 0 && y < Height && x >= 0 && x < Width;
+
+        private bool DFS(int y, int x, int i) {
+            if (i == Word.Length) {
+                return true;
+            }
+
+            if (!Has(y, x) || Board[y][x] == '!' || Word[i] != Board[y][x]) {
+                return false;
+            }
+
+            var tmp = Board[y][x];
+            Board[y][x] = '!';
+
+            foreach (int d in new int[] { 1, -1 }) {
+                if (DFS(y + d, x, i + 1) || DFS(y, x + d, i + 1)) {
+                    return true;
+                }
+            }
+
+            Board[y][x] = tmp;
+            return false;
+        }
+
+        public bool Solve() {
+            for (int y = 0; y < Height; ++y) {
+                for (int x = 0; x < Width; ++x) {
+                    if (DFS(y, x, 0)) {
+                        return true;
+                    }
+                }
+            }
+
+            return false;
+        }
+    }
+
+    public bool Exist(char[][] board, string word) {
+        return new Solver(board, word).Solve();
+    }
+}