FTPrep, 37 Sukodu Solver

TODO: the explanation for the flow

1, recursive function: 3 key points

2, naming is important for logic and debugging, always having a good variable name

3, one-time bug free is left to do, first time is xxx minute and second

代码：

public class Solution {    public void solveSudoku(char[][] board) {        helper(board, 0, 0);        }        private boolean helper(char[][] board, int row, int col){        if(col==9) return helper(board, row+1, 0);        if(row==9) return true;        if(board[row][col]=='.'){            for(int k=1; k<=9; k++){                board[row][col] = (char)('0'+ k);                if(isValid(board, row, col, board[row][col])){                    if(helper(board, row, col+1)) return true;                }                board[row][col]='.';            }            return false;        }        else return helper(board, row, col+1);    }        private boolean isValid(char[][] board, int i, int j, char ch){        for(int col=0; col<board[0].length; col++){            if(col!=j&& ch==board[i][col]) return false;        }        for(int row=0; row<board.length; row++){            if(row!=i&& ch==board[row][j]) return false;        }        for(int row=i/3*3; row<i/3*3 +3; row++){            for(int col=j/3*3; col<j/3*3+3; col++){                if((row!=i || col!=j) && ch==board[row][col]) return false;            }        }        return true;    }}