Problem:
Score : 400 points

Problem Statement

There is a knight - the chess piece - at the origin (0, 0) of a two-dimensional grid.

When the knight is at the square (i, j), it can be moved to either (i+1,j+2) or (i+2, j+1).

In how many ways can the knight reach the square (X, Y)?

Find the number of ways modulo 10^9 + 7.

Constraints

1 \leq X \leq 10^6

1 \leq Y \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X Y

Output

Print the number of ways for the knight to reach (X, Y) from (0, 0), modulo 10^9 + 7.

Sample Input 1

3 3

Sample Output 1

2

There are two ways: (0,0) \to (1,2) \to (3,3) and (0,0) \to (2,1) \to (3,3).

Sample Input 2

2 2

Sample Output 2

0

The knight cannot reach (2,2).

Sample Input 3

999999 999999

Sample Output 3

151840682

Print the number of ways modulo 10^9 + 7.

Solution in Rust (1.15.1) (judge verdict: Accepted)

#![allow(unused_imports)]
use std::cmp::{self, Ord, Ordering};
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

use nyan::BinarySearch;

fn main() {
    let stdin = std::io::stdin();
    let mut sc = nyan::Scanner::new(stdin.lock());

    let x: usize = sc.read();
    let y: usize = sc.read();

    const MOD: usize = 1_000_000_007;

    if (x + y) % 3 != 0 {
        println!("0");
        return;
    }

    //  a + 2b = x
    // 2a +  b = y
    // b = x - y + a
    // 3a = 2y - x
    let (x, y) = (cmp::min(x, y), cmp::max(x, y));
    let d = y - x;
    if d > x {
        println!("0");
        return;
    }

    let a = (2 * y - x) / 3;
    let b = (2 * x - y) / 3;
    if a < 0 || b < 0 {
        println!("0");
        return;
    }

    // fact[n] ≡ n! (mad MOD)
    let mut fact = vec![1; a + b + 2];
    for i in 1..a + b + 2 {
        fact[i] = fact[i - 1] * i % MOD
    }

    // inv[n] = n^(-1) (mod MOD)
    let inv = {
        let mut inv = vec![0; cmp::max(a, b) + 2];
        inv[1] = 1;
        for i in 2..cmp::max(a, b) + 2 {
            inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
        }
        inv
    };

    // fact_inv[n] = (n!)^(-1)
    let fact_inv = {
        let mut fact_inv = vec![0; cmp::max(a, b) + 2];
        fact_inv[0] = 1;
        fact_inv[1] = 1;
        for i in 2..cmp::max(a, b) + 2 {
            fact_inv[i] = fact_inv[i - 1] * inv[i] % MOD;
        }
        fact_inv
    };

    // nCk = n! / (k! * (n - k)!)
    //          n = a + b, k = a
    //     = (a + b)! / (a! * b!)
    let mut ans = fact[a + b];
    ans *= fact_inv[a];
    ans %= MOD;
    ans *= fact_inv[b];
    ans %= MOD;
    println!("{}", ans);
}

#[allow(dead_code)]
mod nyan {
    use std::cmp::*;
    use std::fmt::{Debug, Display};
    use std::io::Read;
    use std::iter::FromIterator;
    use std::str::FromStr;

    pub struct Scanner<R: Read> {
        reader: R,
    }

    impl<R: Read> Scanner<R> {
        pub fn new(reader: R) -> Scanner<R> {
            Scanner { reader: reader }
        }

        fn chars<'a>(&'a mut self) -> Box<Iterator<Item = char> + 'a> {
            Box::new(self.reader.by_ref().bytes().map(|b| b.unwrap() as char))
        }

        // since Rust 1.26
        // fn chars<'a>(&'a mut self) -> impl Iterator<Item = char> + 'a {
        //     self.reader.by_ref().bytes().map(|b| b.unwrap() as char)
        // }

        pub fn next<T: FromIterator<char>>(&mut self) -> T {
            self.chars()
                .skip_while(|c| c.is_whitespace())
                .take_while(|c| !c.is_whitespace())
                .collect()
        }

        pub fn next_line<T: FromIterator<char>>(&mut self) -> T {
            self.chars()
                .skip_while(|c| c.is_whitespace())
                .take_while(|c| !c.is_whitespace() || c == &' ')
                .collect()
        }

        pub fn read<T>(&mut self) -> T
        where
            T: FromStr,
            <T as FromStr>::Err: Debug,
        {
            self.next::<String>().parse().unwrap()
        }

        pub fn read_vec<T, U>(&mut self, n: usize) -> T
        where
            T: FromIterator<U>,
            U: FromStr,
            <U as FromStr>::Err: Debug,
        {
            (0..n).map(|_| self.read()).collect()
        }

        pub fn read_vec2d<T, U, V>(&mut self, m: usize, n: usize) -> T
        where
            T: FromIterator<U>,
            U: FromIterator<V>,
            V: FromStr,
            <V as FromStr>::Err: Debug,
        {
            (0..m).map(|_| self.read_vec(n)).collect()
        }

        pub fn read_line<T: FromIterator<char>>(&mut self) -> T {
            self.next_line()
        }

        pub fn read_lines<T, U>(&mut self, n: usize) -> T
        where
            T: FromIterator<U>,
            U: FromIterator<char>,
        {
            (0..n).map(|_| self.read_line()).collect()
        }
    }

    // C++ like binary search functions (upper_bound, lower_bound, equal_range)
    pub trait BinarySearch<T> {
        fn lower_bound(&self, value: &T) -> usize;
        fn upper_bound(&self, value: &T) -> usize;
        fn equal_range(&self, value: &T) -> (usize, usize) {
            (self.lower_bound(value), self.upper_bound(value))
        }
    }

    impl<T: Ord> BinarySearch<T> for [T] {
        // https://en.cppreference.com/w/cpp/algorithm/lower_bound#Possible_implementation
        fn lower_bound(&self, value: &T) -> usize {
            let mut first = 0;
            let mut last = self.len();
            while last != first {
                let mid = (first + last) / 2;
                match self[mid].cmp(value) {
                    Ordering::Less => first = mid + 1,
                    _ => last = mid,
                }
            }
            first
        }

        // https://en.cppreference.com/w/cpp/algorithm/upper_bound#Possible_implementation
        fn upper_bound(&self, value: &T) -> usize {
            let mut first = 0;
            let mut last = self.len();
            while last != first {
                let mid = (first + last) / 2;
                match self[mid].cmp(value) {
                    Ordering::Greater => last = mid,
                    _ => first = mid + 1,
                }
            }
            first
        }
    }

    #[test]
    pub fn test_binary_search() {
        let data = vec![1, 1, 2, 3, 3, 3, 3, 4, 4, 4, 5, 5, 6];
        assert_eq!(data.lower_bound(&4), 7);
        assert_eq!(data.upper_bound(&4), 10);
    }

    // https://doc.rust-lang.org/std/cmp/struct.Reverse.html
    #[derive(PartialEq, Eq, Debug)]
    pub struct Rev<T>(pub T);

    impl<T: PartialOrd> PartialOrd for Rev<T> {
        fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
            other.0.partial_cmp(&self.0)
        }
    }

    impl<T: Ord> Ord for Rev<T> {
        fn cmp(&self, other: &Self) -> Ordering {
            other.0.cmp(&self.0)
        }
    }

    pub fn gcd(a: u64, b: u64) -> u64 {
        if b == 0 {
            a
        } else {
            gcd(b, a % b)
        }
    }

    pub fn print_vec<T: Display>(v: &[T]) {
        if v.is_empty() {
            return;
        }

        print!("{}", v[0]);
        for i in v.iter().skip(1) {
            print!(" {}", i);
        }
        println!();
    }

    // Extended Euclidean algorithm
    // ax + by = gcd(x, y)
    pub fn extended_gcd(a: u64, b: u64) -> (u64, i64, i64) {
        if b == 0 {
            (a, 1, 0)
        } else {
            let (m, y, x) = extended_gcd(b, a % b);
            (m, x, y - (a / b) as i64 * x)
        }
    }

    pub fn lcm(a: u64, b: u64) -> u64 {
        (a * b) / gcd(a, b)
    }

    pub struct UnionFind {
        n: usize,
        root: Vec<usize>,
        rank: Vec<usize>,
        size: Vec<usize>,
    }

    impl UnionFind {
        pub fn new(n: usize) -> UnionFind {
            UnionFind {
                n: n,
                root: (0..n).collect(),
                rank: vec![0; n],
                size: vec![1; n],
            }
        }

        pub fn root(&mut self, x: usize) -> usize {
            let r = self.root[x];
            if r == x {
                r
            } else {
                self.root[x] = self.root(r);
                self.root[x]
            }
        }

        pub fn merge(&mut self, x: usize, y: usize) {
            let rx = self.root(x);
            let ry = self.root(y);
            if rx == ry {
                return;
            }

            if self.rank[rx] < self.rank[ry] {
                self.root[rx] = ry;
                self.size[ry] += self.size[rx];
            } else {
                self.root[ry] = rx;
                self.size[rx] += self.size[ry];
                if self.rank[rx] == self.rank[ry] {
                    self.rank[rx] += 1;
                }
            }
        }

        pub fn size(&mut self, x: usize) -> usize {
            let rx = self.root(x);
            self.size[rx]
        }
    }
}