解法都在代码里，不懂就留言或者私信，比第一题稍微难点

public static void solveSudoku(char[][] board) {/**定义三个二维数组分别代表行、列、桶（每9个格子）*/boolean[][] rowExists = new boolean[9][10];boolean[][] colExists = new boolean[9][10];boolean[][] bucketExists = new boolean[9][10];/**根据原始board初始化三个是否存在的二维数组*/initArrays(board, rowExists, colExists, bucketExists);process(board, rowExists, colExists, bucketExists, 0, 0);}private static boolean process(char[][] board, boolean[][] rowExists, boolean[][] colExists, boolean[][] bucketExists, int curRow, int curCol) {/**我们是按照行从下到上填的，一行填完才填下一行，如果行越界了，说明正常范围内的行都填完了*/if(curRow == 9) {return true;}/**如果当前的位置可以填，那下个位置填啥，nextRow和nextCol表示下个要填的位置的行和列*/int nextRow = curCol == 8? curRow + 1 : curRow;int nextCol = curCol == 8? 0 : curCol + 1;/**当前位置填过了就继续填下一个*/if(board[curRow][curCol] != '.') {return process(board, rowExists, colExists, bucketExists, nextRow, nextCol);} else {/**当前位置可以填就填，那我们填哪些呢？所有的目前符合的（行列桶都不违规）都可以试试*/for(char c = '1'; c <= '9'; c++) {int num = c - '0';int bucketNum = (curRow/3) * 3 + curCol / 3;/**行、列、桶任意一个存在就不能填*/if(rowExists[curRow][num] || colExists[curCol][num] || bucketExists[bucketNum][num]) {continue;}board[curRow][curCol] = c;/**填完之后行、列、桶做标记*/rowExists[curRow][num] = true;colExists[curCol][num] = true;bucketExists[bucketNum][num] = true;/**如果后面的过程尝试正确就直接返回，否则不处理*/if(process(board, rowExists, colExists, bucketExists, nextRow, nextCol)) {return true;}/**不成功一定要恢复现场*/board[curRow][curCol] = '.';rowExists[curRow][num] = false;colExists[curCol][num] = false;bucketExists[bucketNum][num] = false;}}/**如果中间过程都没有返回，返回false*/return false;}/**遍历board初始化是否存在的数组*/private static void initArrays(char[][] board, boolean[][] rowExists, boolean[][] colExists, boolean[][] bucketExists) {for(int i = 0; i < board.length; i++) {for(int j = 0; j < board[i].length; j++) {if(board[i][j] == '.') {continue;}int num = board[i][j] - '0';int bucketNum = (i / 3) * 3 + j / 3;rowExists[i][num] = true;colExists[j][num] = true;bucketExists[bucketNum][num] = true;}}}

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