If you're using static arrays etc., you can use Custom Invocation to find out how much memory your code takes on Codeforces' servers.

If you are on Linux, you can use POSIX getrusage

#include <sys/resource.h>
#include <sys/time.h>
//above libraries have to be included

int getrusage(int who, struct rusage *usage);

Read the man page to find out how it works.

long   ru_idrss;         /* integral unshared data size */
long   ru_isrss;         /* integral unshared stack size */

These are the 2 elements of the struct you should be looking for.

If you don't need those informations in the program you can use the terminal program (if you use Linux) /usr/bin/time with the -v flag.

/usr/bin/time -v ./a.out <input1 >output1
	Command being timed: "./a.out"
	User time (seconds): 0.10
	System time (seconds): 0.00
	Percent of CPU this job got: 99%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.11
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 29268 <= This is the maximum memory usage of your program
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 0
	Minor (reclaiming a frame) page faults: 1361
	Voluntary context switches: 1
	Involuntary context switches: 3
	Swaps: 0
	File system inputs: 0
	File system outputs: 1136
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0

if u only use arrays, or can figure out the maximum size of the data structure that you are using (vector,map,ect), you can just do math

assuming you declare int a[1000000], it means you declare 1000000 integers, 1 integer = 4 byte so 1000000 integers = 1000000 x 4 = 4000000 bytes = ~4 MB (1 B = ~1000000 MB)

same thing for vectors and others, but vectors are dynamic memory, so you can do it by calculating the worst case ammount of data in the structure