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ACCOUNTS RECEIVABLE, NET - Allowance for doubtful accounts (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Allowance for doubtful accounts:    
Balance at beginning of year ¥ (2,993) ¥ (3,535)
Addition (7,857) (3,220)
Written off 4,603 3,762
Balance at end of year ¥ (6,247) ¥ (2,993)