Write a program to solve a Sudoku puzzle by filling the empty cells.
A sudoku solution must satisfy all of the following rules:
Each of the digits 1-9 must occur exactly once in each row.
Each of the digits 1-9 must occur exactly once in each column.
Each of the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.
The '.' character indicates empty cells.
public class Solution {
public void solveSudoku(char[][] board) {
dfs(board,0);
}
private boolean dfs(char[][] board, int d) {
if (d==81) return true;
int i=d/9, j=d%9;
if (board[i][j]!='.') return dfs(board,d+1);
boolean[] flag=new boolean[10];
validate(board,i,j,flag);
for (int k=1; k<=9; k++) {
if (flag[k]) {
board[i][j]=(char)('0'+k);
if (dfs(board,d+1)) return true;
}
}
board[i][j]='.';
return false;
}
private void validate(char[][] board, int i, int j, boolean[] flag) {
Arrays.fill(flag,true);
for (int k=0; k<9; k++) {
if (board[i][k]!='.') flag[board[i][k]-'0']=false;
if (board[k][j]!='.') flag[board[k][j]-'0']=false;
int r=i/3*3+k/3;
int c=j/3*3+k%3;
if (board[r][c]!='.') flag[board[r][c]-'0']=false;
}
}
}
