Sudoku Solver

public class Solution {
    public void solveSudoku(char[][] board) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        solveSudokuRecursive(board);
    }
    private boolean solveSudokuRecursive(char[][] board) {
        int[] pairs = getFirstEmpty(board);
        if (pairs[0] == -1 && pairs[1] == -1)
            return true;
        for (int i = 1; i <= 9; i++) {
            board[pairs[0]][pairs[1]] = (char) (i + '0');
            if (isValid(board, pairs[0], pairs[1])
                    && solveSudokuRecursive(board)) {
                return true;
            }
            board[pairs[0]][pairs[1]] = '.';
        }
        return false;
    }

    private int[] getFirstEmpty(char[][] board) {
        int[] pairs = null;
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.') {
                    pairs = new int[] { i, j };
                    return pairs;
                }
            }
        }
        pairs = new int[] { -1, -1 };
        return pairs;
    }


    private boolean isValid(char[][] board, int x, int y) {

        for (int i = 0; i < 9; i++) {
            if (i != y && board[x][i] == board[x][y])
                return false;

            if (i != x && board[i][y] == board[x][y])
                return false;
        }

        int xIdx = (x / 3) * 3;
        int yIdx = (y / 3) * 3;
        for (int m = 0; m < 3; m++) {
            for (int n = 0; n < 3; n++) {
                if(m + xIdx != x && n + yIdx != y && board[m + xIdx][n + yIdx] == board[x][y])
                    return false;
            }
        }
        return true;
    }
}