题干：

代码：

class Solution {
public:
    bool backtracking(vector<vector<char>>& board){
        for(int i = 0; i < board.size(); i++){
            for(int j = 0; j < board[0].size(); j++){
                if(board[i][j] == '.'){
                    for(char k = '1'; k <= '9'; k++){
                        if(isvalid(board, k, i, j)){
                            board[i][j] = k;
                            if(backtracking(board))return true;
                            board[i][j] = '.';
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }
    bool isvalid(vector<vector<char>> &board, char val, int row, int col){
        for(int i = 0; i < 9; i++){
            if(board[i][col] == val)return false;
        }
        for(int j = 0; j < 9; j++){
            if(board[row][j] == val)return false;
        }
        int startrow = (row / 3) * 3;
        int startcol = (col / 3) * 3;
        for(int i = startrow; i < startrow + 3; i++){
            for(int j = startcol; j < startcol + 3; j++){
                if(board[i][j] == val)return false;
            }
        }
        return true;
    }
    void solveSudoku(vector<vector<char>>& board) {
        backtracking(board);
    }
};

注意：

bool backtracking(vector<vector<char>>& board)

for(int i = 0; i < board.size(); i++){//遍历行
    for(int j = 0; j < board[0].size(); j++){//遍历列

if(backtracking(board))return true;

for(char k = '1'; k <= '9'; k++){
    if(isvalid(board, k, i, j)){
        board[i][j] = k;
        if(backtracking(board))return true;
        board[i][j] = '.';
    }
}
return false;//false对应的必须是尝试9个数字都不行的结果

int startrow = (row / 3) * 3;
int startcol = (col / 3) * 3;
for(int i = startrow; i < startrow + 3; i++){
    for(int j = startcol; j < startcol + 3; j++){
        if(board[i][j] == val)return false;
    }
}//小9格检验

对使用bool的解释:思路同路径总和，void是举出所有情况，bool是只取一个符合条件的就可。

为什么能够返回填入数字后的结果————因为传入的是地址。