In this problem, you have to maintain a dynamic set of numbers which support the two fundamental operations
INSERT(S,x): if x is not in S, insert x into S
DELETE(S,x): if x is in S, delete x from S
and the two type of queries
K-TH(S) : return the k-th smallest element of S
COUNT(S,x): return the number of elements of S smaller than x
Line 1: Q (1 <= Q <= 200000), the number of operations
In the next Q lines, the first token of each line is a character I, D, K or C meaning that the corresponding operation is INSERT, DELETE, K-TH or COUNT, respectively, following by a whitespace and an integer which is the parameter for that operation.
If the parameter is a value x, it is guaranteed that 0 <= |x| <= 109. If the parameter is an index k, it is guaranteed that 1 <= k <= 10^9.
For each query, print the corresponding result in a single line. In particular, for the queries K-TH, if k is larger than the number of elements in S, print the word 'invalid'.
I can't solve this problem with trie
someone can help me !!!!