I am not allocating any memory in heap, everything is defined globally, why do I get Memory Limit Exceeded error on test 5? Why this error is not given for test 1? I'm not even using STL.

It also happened to me solving another problem(1220. Stacks), I got memory limit exceeded on test 10, same situation as here. Does IO operations use memory? I assume I get this error as test-cases get larger in size... (I also tried disabling IO buffering)

Solved. It was a problem with my DSU function.