I am using a solution, where I store all the doors in a BST, and if door k is to be deleted then I delete the kth smallest element in the BST. Same for lookup, I search the kth smallest and print the number.

In test case 2 it is showing memory limit exceeded with only some 66-69KB memory usages. But the question says 64MB memory is allocated. Then why is this happening?
(I am using C++ to code this.)