递归地按螺旋顺序打印矩阵

def spiralOrder(matrix):
    if not matrix:
        return []
    
    def helper(matrix, row_start, col_start, row_end, col_end):
        if row_start > row_end or col_start > col_end:
            return []
        
        if row_start == row_end:
            return matrix[row_start][col_start:col_end+1]
        
        if col_start == col_end:
            return [matrix[i][col_start] for i in range(row_start, row_end+1)]
        
        res = []
        for j in range(col_start, col_end+1):
            res.append(matrix[row_start][j])
        for i in range(row_start+1, row_end+1):
            res.append(matrix[i][col_end])
        for j in range(col_end-1, col_start-1, -1):
            res.append(matrix[row_end][j])
        for i in range(row_end-1, row_start, -1):
            res.append(matrix[i][col_start])
        
        return res + helper(matrix, row_start+1, col_start+1, row_end-1, col_end-1)
    
    return helper(matrix, 0, 0, len(matrix)-1, len(matrix[0])-1)