37. Sudoku Solver

class Solution {
    public void solveSudoku(char[][] board) {
        boolean[][][] used=new boolean[3][9][9];
        for(int i=0;i<9;i++)
            for(int j=0;j<9;j++)
                if(board[i][j]!='.')
                {
                    int idx=(int)board[i][j]-'1';
                    used[0][i][idx]=true;
                    used[1][j][idx]=true;
                    int k=i/3*3+j/3;
                    used[2][k][idx]=true;
                }
        dfs(board,used);
    }
    private boolean dfs(char[][] board, boolean[][][] used)
    {
        int row=-1;
        int col=-1;
        boolean found=false;
        for(int i=0;i<9;i++)
            for(int j=0;j<9;j++)
                if(board[i][j]=='.'&&found==false)
                {
                    row=i;
                    col=j;
                    found=true;
                }
        if(found==false)
            return true;
        int k=row/3*3+col/3;
        for(int idx=0;idx<9;idx++)
            if(used[0][row][idx]==false&&used[1][col][idx]==false&&used[2][k][idx]==false)
            {
                board[row][col]=(char)('1'+idx);
                used[0][row][idx]=true;
                used[1][col][idx]=true;
                used[2][k][idx]=true;
                if(dfs(board,used)==true)
                    return true;
                board[row][col]='.';
                used[0][row][idx]=false;
                used[1][col][idx]=false;
                used[2][k][idx]=false;
            }
        return false;
    }
}