class Solution { public boolean solveSudoku(char[][] board) { return solve(board); } private boolean solve(char[][] board) { for (int i = 0; i < 9; i++) { for (int j = 0; j < 9; j++) { if (board[i][j] == '.') { for (char c = '1'; c <= '9'; c++) { if (isValid(board, i, j, c)) { board[i][j] = c; if (solve(board)) return true; board[i][j] = '.'; // backtrack } } return false; } } } return true; } private boolean isValid(char[][] board, int row, int col, char c) { for (int i = 0; i < 9; i++) { // row check if (board[row][i] == c) return false; // column check if (board[i][col] == c) return false; // 3x3 box check int r = 3 * (row / 3) + i / 3; int c2 = 3 * (col / 3) + i % 3; if (board[r][c2] == c) return false; } return true; } }