Write a program to solve a Sudoku puzzle by filling the empty cells.
Empty cells are indicated by the character '.'
.
You may assume that there will be only one unique solution.
A sudoku puzzle...
...and its solution numbers marked in red.
暴力DFS填了,可以预处理找出每行、每列、每9小格“能用的”集合,尝试填数的时候用一个删一个,不过这样回溯也慢。
public class Solution {
boolean[][] rowused=new boolean[9][10];
boolean[][] columnused=new boolean[9][10];
boolean[][] gridused=new boolean[9][10];
HashSet<Integer>[] rowcanuse=new HashSet[9];
HashSet<Integer>[] columncanused=new HashSet[9];
HashSet<Integer>[] gridcanused=new HashSet[9];
public void solveSudoku(char[][] board)
{
for(int i=0;i<9;i++)
for(int j=0;j<9;j++)
{
char cij=board[i][j];
if(cij!='.')
{
rowused[i][cij-'0']=true;
columnused[j][cij-'0']=true;
gridused[i/3*3+j/3][cij-'0']=true;
}
}
for(int i=0;i<9;i++)
{
rowcanuse[i]=new HashSet<>(10);
columncanused[i]=new HashSet<>(10);
gridcanused[i]=new HashSet<>(10);
}
for(int i=0;i<9;i++)
for(int j=1;j<=9;j++)
{
if(!rowused[i][j])
rowcanuse[i].add(j);
if(!columnused[i][j])
columncanused[i].add(j);
if(!gridused[i][j])
gridcanused[i].add(j);
}
dfs(board, 0, 0);
}
public boolean dfs(char[][] board,int i,int j)
{
if(i==8&&j==8)
{
if(!rowcanuse[i].isEmpty())
{
Iterator<Integer> it=rowcanuse[i].iterator();
board[i][j]=(char) (it.next()+'0');
}
return true;
}
char c=board[i][j];
if(c=='.')
{
for(int k=1;k<=9;k++)
if(rowcanuse[i].contains(k)&&columncanused[j].contains(k)&&gridcanused[i/3*3+j/3].contains(k))
{
board[i][j]=(char) (k+'0');
rowcanuse[i].remove(k);
columncanused[j].remove(k);
gridcanused[i/3*3+j/3].remove(k);
boolean success=false;
if(j==8)
success=dfs(board, i+1, 0);
else {
success=dfs(board, i, j+1);
}
if(success)
return true;
rowcanuse[i].add(k);
columncanused[j].add(k);
gridcanused[i/3*3+j/3].add(k);
board[i][j]='.';
}
}
else
{
if(j==8)
return dfs(board, i+1, 0);
else {
return dfs(board, i, j+1);
}
}
return false;
}
}