看了大佬的題解，茅塞頓開

public void solveSudoku(char[][] board) {
        if(board == null || board.length != 9 || board[0] == null || board[0].length != 9){
            return;
        }
        backtrack(board,0,0);


    }
    public boolean backtrack(char[][] board,int i ,int j){
        int m = 9,n = 9;
        
 
if(j == n){
            return backtrack(board,i+1,0);
        }
        if(i == m){
            return true;
        }
        if(board[i][j] != '.'){
            return backtrack(board,i,j+1);
        }
        for(char ch = '1';ch <= '9';ch++){
            if(!isValid(board,i,j,ch)){
                
 
continue;
            }
            board[i][j] = ch;
            if(backtrack(board,i,j+1)){
                return true;
            }
            board[i][j] = '.';
        }
        return false;
    }

    private boolean isValid(char[][] board, int r, int c, char n) {
        for(int i = 0;i<9;i++){
            
 
if(board[r][i] == n){
                return false;
            }
            if(board[i][c] == n){
                return false;
            }
            if(board[(r/3)*3+i/3][(c/3)*3+i%3] == n){
                return false;
            }
        }
        return true;
    }

——2020.6.30