Equilateral numbers

use std::str::FromStr;
use std::fmt::Debug;
use std::collections::HashSet;

fn read<T: FromStr>() -> Vec<T>where <T as FromStr>::Err: Debug  {
    let stdin = std::io::stdin();
    let line = stdin.lines().next().unwrap().unwrap();
    line.split_whitespace().map(T::from_str).map(|x| x.unwrap()).collect()
}

fn main() {
    let n = read::<usize>()[0];

    let nums = (1..=1_501_000).map(|n| n * (n + 1) / 2).collect::<HashSet<_>>();
    
    if nums.contains(&n) {
        println!("1");
    } else if nums.iter().any(|s| nums.contains(&n.wrapping_sub(*s))) {
        println!("2");
    } else {
        println!("3");
    }
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED