Sudoku Solver

• https://leetcode.com/problems/sudoku-solver/

Time complexity: O(n ^ m) where n is the number of possibilities for each square (i.e., 9 in classic Sudoku) and m is the number of spaces that are blank.

public void solveSudoku(char[][] board) {
    sudokuHeper(board);
}

public boolean sudokuHeper(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 (sudokuHeper(board)) { // with the updated board, try if it is a vlid one.
                            return true;
                        } else {
                            board[i][j] = '.';
                        }
                    }
                }
                // after we try every single possible solution
                // if we coudln't find it, return false
                return false;
            }
        }
    }
    return true;
}

private boolean isValid(char[][] board, int m, int n, char val) {
    // check rows
    for (int j = 0; j < board[0].length; j++) {
        if (board[m][j] == val)
            return false;
    }

    // check cols
    for (int i = 0; i < board.length; i++) {
        if (board[i][n] == val)
            return false;
    }

    // check regions
    int I = m / 3;
    int J = n / 3;

    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            if (val == board[I * 3 + i][J * 3 + j]) return false;
        }
    }
    return true;
}