Problem:
Score : 700 points

Problem Statement

There is a tree with N vertices, numbered 1 through N.
The i-th of the N-1 edges connects vertices a_i and b_i.

Currently, there are A_i stones placed on vertex i.
Determine whether it is possible to remove all the stones from the vertices by repeatedly performing the following operation:

Select a pair of different leaves. Then, remove exactly one stone from every vertex on the path between those two vertices.
Here, a leaf is a vertex of the tree whose degree is 1, and the selected leaves themselves are also considered as vertices on the path connecting them.

Note that the operation cannot be performed if there is a vertex with no stone on the path.

Constraints

2 ≦ N ≦ 10^5

1 ≦ a_i,b_i ≦ N

0 ≦ A_i ≦ 10^9

The given graph is a tree.

Input

The input is given from Standard Input in the following format:

N
A_1 A_2 … A_N
a_1 b_1
:
a_{N-1} b_{N-1}

Output

If it is possible to remove all the stones from the vertices, print YES. Otherwise, print NO.

Sample Input 1

5
1 2 1 1 2
2 4
5 2
3 2
1 3

Sample Output 1

YES

All the stones can be removed, as follows:

Select vertices 4 and 5. Then, there is one stone remaining on each vertex except 4.

Select vertices 1 and 5. Then, there is no stone on any vertex.

Sample Input 2

3
1 2 1
1 2
2 3

Sample Output 2

NO

Sample Input 3

6
3 2 2 2 2 2
1 2
2 3
1 4
1 5
4 6

Sample Output 3

YES

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

#![allow(unused_imports)]
#![allow(dead_code)]
use std::cmp::*;
use std::collections::*;

const NOTHING: usize = std::usize::MAX;

fn main() {
    let n = readln();
    let cs: Vec<i64> = readln();
    let mut gr: Vec<Vec<usize>> = vec![vec![];n];
    for _ in 0..n - 1 {
        let (a, b): (usize, usize) = readln();
        gr[a - 1].push(b - 1);
        gr[b - 1].push(a - 1);
    }
    let gr = gr;

    if n == 2 {
        if cs[0] == cs[1] {
            println!("YES");
        } else {
            println!("NO");
        }
        return;
    }
    let mut root = 0;
    for v in 0..n {
        if gr[v].len() > 1 {
            root = v;
            break;
        }
    }

    if let Some(0) = dfs(&gr, root, NOTHING, &cs) {
        println!("YES");
    } else {
        println!("NO");
    }
}

fn dfs(gr: &Vec<Vec<usize>>, v: usize, parent: usize, cs: &Vec<i64>) -> Option<i64> {
    let len = gr[v].len() - 1;
    if len == 0 {
        return Some(cs[v]);
    }

    let mut children: Vec<i64> = Vec::with_capacity(len);
    for &nv in &gr[v] {
        if nv == parent {
            continue;
        }
        if let Some(c) = dfs(gr, nv, v, cs) {
            children.push(c)
        } else {
            return None;
        }
    }

    let s: i64 = children.iter().fold(0, |acc, &x| acc + x);
    let m: i64 = children.iter().fold(0, |acc, &x| max(acc, x));

    let t = 2 * cs[v] - s;

    if t >= 0 && (s - t) <= 2 * min(s / 2, s - m) && s - t >= 0 {
        Some(t)
    } else {
        None
    }

}

fn get_line() -> String {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    buf.trim_right().to_string()
}

fn get_chars() -> Vec<char> {
    get_line().chars().collect()
}

fn readln<T: Read>() -> T {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).unwrap();
    T::read(buf.trim_right())
}

trait Read {
    fn read(s: &str) -> Self;
}

macro_rules! read_impl{
    ($($t:ty)*) => ($(
        impl Read for $t {
            fn read(s: &str) -> $t{
                s.parse().unwrap()
            }
        }
    )*)
}
read_impl! { usize u8 u16 u32 u64 isize i8 i16 i32 i64 f32 f64 }

impl<T: Read> Read for Vec<T> {
    fn read(s: &str) -> Self {
        s.split_whitespace().map(T::read).collect()
    }
}

impl<A: Read, B: Read> Read for (A, B) {
    fn read(s: &str) -> Self {
        let tokens: Vec<_> = s.split_whitespace().collect();
        (A::read(tokens[0]), B::read(tokens[1]))
    }
}

impl<A: Read, B: Read, C: Read> Read for (A, B, C) {
    fn read(s: &str) -> Self {
        let tokens: Vec<_> = s.split_whitespace().collect();
        (A::read(tokens[0]), B::read(tokens[1]), C::read(tokens[2]))
    }
}