SudokuSolver.java

public class Solution {

  public static boolean solveSudoku(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 (solveSudoku(board))
                return true;
              else
                board[i][j] = '.';
            }
          }

          return false;
        }
      }
    }
    return true;
  }

  public static boolean isValid(char[][] board, int row, int col, char c) {
    for (int i = 0; i < 9; i++) {
      if (board[i][col] == c)
        return false;

      if (board[row][i] == c)
        return false;

      if (board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] == c)
        return false;
    }
    return true;
  }

  public static void main(String[] args) {

   char[][] board= {
                    {'9', '5', '7', '.', '1', '3', '.', '8', '4'},
                    {'4', '8', '3', '.', '5', '7', '1', '.', '6'},
                    {'.', '1', '2', '.', '4', '9', '5', '3', '7'},
                    {'1', '7', '.', '3', '.', '4', '9', '.', '2'},
                    {'5', '.', '4', '9', '7', '.', '3', '6', '.'},
                    {'3', '.', '9', '5', '.', '8', '7', '.', '1'},
                    {'8', '4', '5', '7', '9', '.', '6', '1', '3'},
                    {'.', '9', '1', '.', '3', '6', '.', '7', '5'},
                    {'7', '.', '6', '1', '8', '5', '4', '.', '9'}
                    };
    solveSudoku(board);

    for (int i = 0; i < 9; i++) {
      for (int j = 0; j < 9; j++)
        System.out.print(board[i][j] + " ");
      System.out.println();
    }
  }
}

/***
Test Case 1 : 
Input
board =
[["5","3",".",".","7",".",".",".","."],["6",".",".","1","9","5",".",".","."],[".","9","8",".",".",".",".","6","."],["8",".",".",".","6",".",".",".","3"],["4",".",".","8",".","3",".",".","1"],["7",".",".",".","2",".",".",".","6"],[".","6",".",".",".",".","2","8","."],[".",".",".","4","1","9",".",".","5"],[".",".",".",".","8",".",".","7","9"]]
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Output
[["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
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Expected
[["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
 */