Fast I/O, Judges & Debugging

8 cards, about a minute. Read them the night before an interview, or right after the chapter to lock it in.

1. ◆what it is

   A judge runs your program once per test: input file → stdin → your code → stdout → checker. It only sees what you print.

   #1
2. ↯use it when

   Stdin mode with 10⁵+ tokens → BufferedReader + StringTokenizer in, StringBuilder or PrintWriter out. Small input → Scanner is fine.

   #2
3. { }template

   PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out)); … out.println(ans); out.flush();

   #3
4. O( )cost

   Rule of thumb: ~10⁸ simple steps per second. n = 10⁵: O(n²) = 10¹⁰ → TLE. O(n log n) ≈ 1.7 × 10⁶ → fine.

   #4
5. !trap

   PrintWriter without flush() prints nothing → WA. nextInt() then nextLine() returns "". Debug prints on stdout → WA.

   #5
6. ✦pro tip

   Verdicts are diagnoses: WA = output, TLE = complexity or slow I/O, RE = an exception, MLE = memory, CE = compile.

   #6
7. !trap

   AIOOBE = bad index · NPE = null · NumberFormat = bad parse · Arithmetic = / by zero · CME = edit in for-each · StackOverflow = deep recursion.

   #7
8. ✦pro tip

   Stuck on WA? Samples → edge cases → stress test vs brute force → shrink the failing input → dry-run it.

   #8

Shaky on a card? Open the chapter or flip its recall cards.