leetcode 37 Sudoku Solver java

求数独，只要求做出一个答案就可以。

刚开始对题意理解错误，以为答案是唯一的， 所以做了很久并没有做出来，发现答案不唯一之后，使用回溯。（还是借鉴了一下别人）

public class Solution {
    public void solveSudoku(char[][] board) {
        HashSet[] hashset = new HashSet[27];
		for (int i = 0; i < 27; i++)
			hashset[i] = new HashSet<Character>();
		for (int i = 0; i < 9; i++) {
			for (int j = 0; j < 9; j++) {
				char Char = board[i][j];
				if (Char != ‘.‘) {
						hashset[i].add(Char);
						hashset[9 + j].add(Char);
						hashset[18 + (i / 3) * 3 + j / 3].add(Char);
				}
			}
		}
		int flag = 0;
		char[][][] num = null ;
		while ( flag == 0) {
			flag = 1;
			num = new char[9][9][9];
			for (int i = 0; i < 9; i++) {// i代表第i个hashset
				for (int j = 1; j < 10; j++) {// j代表1-9
					char ch = (char) (j + ‘0‘);
					int[] test = new int[2];
					if (!hashset[i].contains(ch)) {
						test[0] = 0;
						for (int k = 0; k < 9; k++) {
							char Ch = board[i][k];
							if (Ch == ‘.‘) {
								if (!hashset[9 + k].contains(ch) && !hashset[18 + (i / 3) * 3 + k / 3].contains(ch)) {
									addNum(num, i, k, ch);
									test[0]++;
									test[1] = k;
								}
							}
						}
					}
					if (test[0] == 1) {
						board[i][test[1]] = ch;
						hashset[i].add(ch);
						flag = 0;
						hashset[9 + test[1]].add(ch);
						hashset[18 + (i / 3) * 3 + test[1] / 3].add(ch);
					}

				}
			}

			for (int qq = 0; qq < 9 && flag == 1; qq++) {
				for (int j = 0; j < 9 && flag == 1; j++) {
					if (getlen(num[qq][j]) == 1) {
						char ch = num[qq][j][0];
						board[qq][j] = ch;
						flag = 0;
						hashset[qq].add(ch);
						hashset[9 + j].add(ch);
						hashset[18 + (qq / 3) * 3 + j / 3].add(ch);
					}

				}
			}

		}
        solve(board);
    }

    public boolean solve(char[][] board){
        for(int i = 0; i < board.length; i++){
            for(int j = 0; j < board[0].length; j++){
                if(board[i][j] == ‘.‘){
                    for(char c = ‘1‘; c <= ‘9‘; c++){//trial. Try 1 through 9 for each cell
                        if(isValid(board, i, j, c)){
                            board[i][j] = c; //Put c for this cell

                            if(solve(board))
                                return true; //If it‘s the solution return true
                            else
                                board[i][j] = ‘.‘; //Otherwise go back
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }

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

        for(int col = 0; col < 9; col++)
            if(board[i][col] == c)
                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(board[row][col] == c)
                    return false;
        return true;
    }
    public static int getlen(char[] num) {
		int len = 0;
		for (int i = 0; i < 9; i++) {
			if (num[i] < ‘1‘ || num[i] > ‘9‘) {
				return len;
			} else
				len++;
		}
		return len;
	}

	public static void addNum(char[][][] num, int num1, int num2, char ch) {
		for (int i = 0; i < 9; i++) {
			if (num[num1][num2][i] < ‘0‘ || num[num1][num2][i] > ‘9‘) {
				num[num1][num2][i] = ch;
				break;
			}

		}

	}
}

回溯法还是比较简单的，就是在实现的时候，如果想要提高运行的速度和空间，那么需要费一些心思来考虑。

附上借鉴的代码

public class Solution {
    public void solveSudoku(char[][] board) {
        if(board == null || board.length == 0)
            return;
        solve(board);
    }

    public boolean solve(char[][] board){
        for(int i = 0; i < board.length; i++){
            for(int j = 0; j < board[0].length; j++){
                if(board[i][j] == ‘.‘){
                    for(char c = ‘1‘; c <= ‘9‘; c++){//trial. Try 1 through 9 for each cell
                        if(isValid(board, i, j, c)){
                            board[i][j] = c; //Put c for this cell

                            if(solve(board))
                                return true; //If it‘s the solution return true
                            else
                                board[i][j] = ‘.‘; //Otherwise go back
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }

    public boolean isValid(char[][] board, int i, int j, char c){
        //Check colum
        for(int row = 0; row < 9; row++)
            if(board[row][j] == c)
                return false;

        //Check row
        for(int col = 0; col < 9; col++)
            if(board[i][col] == c)
                return false;

        //Check 3 x 3 block
        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(board[row][col] == c)
                    return false;
        return true;
    }
}