37. Sudoku Solver.cpp

37. Sudoku Solver.cpp

@@ -0,0 +1,42 @@
+class Solution {
+public:
+    void solveSudoku(vector<vector<char>>& board) {
+        backtrack(board);
+    }
+
+private:
+    bool backtrack(vector<vector<char>>& board) {
+        for (int r = 0; r < 9; r++) {
+            for (int c = 0; c < 9; c++) {
+                if (board[r][c] == '.') {
+                    for (char ch = '1'; ch <= '9'; ch++) {
+                        if (isValid(board, r, c, ch)) {
+                            board[r][c] = ch;
+                            if (backtrack(board)) return true; // continue with this placement
+                            board[r][c] = '.'; // undo if failed
+                        }
+                    }
+                    return false; // no valid digit, backtrack
+                }
+            }
+        }
+        return true; // all cells filled
+    }
+
+    bool isValid(vector<vector<char>>& board, int r, int c, char ch) {
+        // Row & Column check
+        for (int i = 0; i < 9; i++) {
+            if (board[r][i] == ch) return false;
+            if (board[i][c] == ch) return false;
+        }
+        // 3x3 sub-box check
+        int startRow = (r / 3) * 3;
+        int startCol = (c / 3) * 3;
+        for (int i = 0; i < 3; i++) {
+            for (int j = 0; j < 3; j++) {
+                if (board[startRow + i][startCol + j] == ch) return false;
+            }
+        }
+        return true;
+    }
+};