Performant Code

How to write code that won't timeout on large inputs.

Think About Scale First

Before writing code, ask: how big is the data?

Data size	Approach
< 1 MB	Load into memory, any approach works
1-100 MB	Load into memory, but use efficient algorithms
100 MB - 1 GB	Stream/mmap, avoid loading entirely into memory
> 1 GB	Streaming only, chunk-based processing

I/O Optimization

Large files

• mmap (C:

  mmap()

  , Python:

  mmap.mmap()

  ) — map file into memory, OS handles paging
• Buffered binary reads —

  fread()

  in C,

  open(f, 'rb').read(chunk)

  in Python
• NEVER read a 500MB file line-by-line with

  fgets()

  when you need random access

Writing output

• Buffer writes — don't call

  write()

  for every byte
• Use

  fwrite()

  or

  sys.stdout.buffer.write()

  for binary output
• Flush only when needed

Algorithm Complexity

• O(n) beats O(n log n) beats O(n²) — always
• Nested loops on large data = timeout. Restructure to single pass + hash map
• Sorting is O(n log n) — only sort if you need to
• Use hash maps/sets for lookup instead of linear search
• Pre-compute what you can outside loops

Language-Specific Tips

C

• Use

  mmap()

  for large file access

• -O2

  or

  -O3

  for compiler optimizations
• Avoid

  malloc()

  /

  free()

  in tight loops — pre-allocate
• Use

  memcpy()

  instead of byte-by-byte copying
• Integer arithmetic > floating point when possible

Python

• Use

  numpy

  for numerical work (100x faster than pure Python loops)

• collections.Counter

  ,

  defaultdict

  — avoid manual counting
• List comprehensions > explicit loops

• struct.unpack()

  for binary parsing

• subprocess.run()

  >

  os.system()

• For heavy computation: consider writing a small C program instead

General

• Profile before optimizing — find the actual bottleneck
• If a program hangs, it's likely: infinite loop, deadlock, or I/O bound on huge data
• If a program is slow, check: algorithm complexity, I/O pattern, memory allocation

Constraints Awareness

• If the task says "< 5000 bytes" — count your bytes, use

  wc -c

• If there's a time limit — test with actual data, not toy inputs
• If there's a memory limit — don't load everything into RAM
• Always verify constraints BEFORE declaring done