[DFS] move solution to Sudoku Solver to DFS

DFS/SudokuSolver.swift

@@ -0,0 +1,70 @@
+/**
+ * Question Link: https://leetcode.com/problems/sudoku-solver/
+ * Primary idea: Iterate through the whole matrix, try to fill out empty space with all 
+ *			     possible cases and check the vaildity
+ *
+ * Time Complexity: O(n^4), Space Complexity: O(1)
+ */
+
+ class SudokuSolver {
+    func solveSudoku(_ board: inout [[Character]]) {
+        guard board.count != 0 || board[0].count != 0 else {
+            return
+        }
+        helper(&board)
+    }
+
+    private func helper(_ board: inout [[Character]]) -> Bool {
+        let m = board.count, n = board[0].count
+
+        for i in 0 ..< m {
+            for j in 0 ..< n {
+                if board[i][j] == "." {
+                    for num in 1 ... 9 {
+                        if isValid(board, i, j, Character(String(num))) {
+                            board[i][j] = Character(String(num))
+
+                            if helper(&board) {
+                                return true
+                            } else {
+                                board[i][j] = "."
+                            }
+                        }
+                    }
+                    return false
+                }
+            }
+        }
+
+        return true
+    }
+
+    private func isValid(_ board: [[Character]], _ i: Int, _ j: Int, _ num: Character) -> Bool {
+        let m = board.count, n = board[0].count
+
+        // check row
+        for x in 0 ..< n {
+            if board[i][x] == num {
+                return false
+            }
+        }
+
+        // check col
+        for y in 0 ..< m {
+            if board[y][j] == num {
+                return false
+            }
+        }
+
+        // check square
+        for x in i / 3 * 3 ..< i / 3 * 3 + 3 {
+            for y in j / 3 * 3 ..< j / 3 * 3 + 3 {
+                if board[x][y] == num {
+                    return false
+                }
+            }
+        }
+
+        return true
+    }
+}