Rust - Fast I/O

When solving a P.S. problem, the following simple code can produce TLE (time limit exceeded) error. This is due to rust I/O flushing the buffer every time a value is printed using println macro.

use std::io::{self};

fn main() {
	let mut n = String::new();
	io::stdin().read_line(&mut n).unwrap();
	let n: i32 = n.trim().parse().unwrap();
	
	for i in 0..n {
		println!("{}", i);
	}
}

Instead of printing a value every time, we need to save it in the buffer and output the whole data at the end. We can achieve this by using BufWrite.

use std::io::{self, Write, BufWriter};

fn main() {
	let mut n = String::new();
	io::stdin().read_line(&mut n).unwrap();
	let n: i32 = n.trim().parse().unwrap();

	let stdout = io::stdout(); // 1
	let mut output = BufWriter::new(stdout.lock()); // 2

	for i in 0..n {
		writeln!(output, "{}", i).unwrap(); // 3
	}
}

In step 1 and 2, we declared a BufWriter type variable name output. Any printed data will be stored in this buffer.

In step 3, we used writeln macro instead of println.

This will reduce the number of system calls and increase the performance of your code.