37. Sudoku Solver

Write a program to solve a Sudoku puzzle by filling the empty cells.

A sudoku solution must satisfy all of the following rules:

1. Each of the digits 1-9 must occur exactly once in each row.
2. Each of the digits 1-9 must occur exactly once in each column.
3. Each of the the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.

Empty cells are indicated by the character '.'.

回溯算法，牛逼

class Solution {
   public void solveSudoku(char[][] board) {
    solver(board);
}
private boolean solver(char[][] board) {
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++) {
            if (board[i][j] == '.') {
                char count = '1';
                while (count <= '9') {
                    if (isValid(i, j, board, count)) {
                        board[i][j] = count;
                        if (solver(board)) {
                            return true;
                        } else {
                            //下一个位置没有数字，就还原，然后当前位置尝试新的数
                            board[i][j] = '.';
                        }
                    }
                    count++;
                }
                return false;
            }
        }
    }
    return true;
}

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

    for (int i = 0; i < 9; i++) {
        if (board[i][col] == c) {
            return false;
        }
    }

    int start_row = row / 3 * 3;
    int start_col = col / 3 * 3;
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            if (board[start_row + i][start_col + j] == c) {
                return false;
            }
        }

    }
    return true;
}

}

 二刷现在理解了为什么要row/3*3，比如说row = 8，row/3*3 = 6，那这个block就是从6开始刀6，7，8，col也一样。