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 C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using CompLib.Util;

public class Program
{
    private int N;
    private long[] A;
    private List<int>[] E;

    public void Solve()
    {
        var sc = new Scanner();
        N = sc.NextInt();
        A = sc.LongArray();
        E = new List<int>[N];
        for (int i = 0; i < N; i++)
        {
            E[i] = new List<int>();
        }

        for (int i = 0; i < N - 1; i++)
        {
            int a = sc.NextInt() - 1;
            int b = sc.NextInt() - 1;
            E[a].Add(b);
            E[b].Add(a);
        }

        // 0根

        // 葉 繋がってる頂点以下

        // 葉以外 連結頂点の和/2 以下

        long o;
        if (!Go(0, -1, out o))
        {
            Console.WriteLine("NO");
            return;
        }

        Console.WriteLine(o == 0 ? "YES" : "NO");
    }

    bool Go(int cur, int par, out long t)
    {
        long sum = 0;
        long max = long.MinValue;
        foreach (int to in E[cur])
        {
            if (to == par) continue;
            long c;
            if (!Go(to, cur, out c))
            {
                t = -1;
                return false;
            }

            // if(cur == 0)Console.WriteLine(c);
            max = Math.Max(max, c);
            sum += c;
        }

        if (E[cur].Count == 1)
        {
            if (par != -1)
            {
                t = A[cur];
                return true;
            }
            else
            {
                if (A[cur] == sum)
                {
                    t = 0;
                    return true;
                }
                else
                {
                    t = -1;
                    return false;
                }
            }
        }
        else
        {
            // 子を通る回数合計 sum

            // 子同士通る = L
            // 親と子を通る = T

            // T + L = A
            // 2L + T = sum
            // 2A - sum = T
            t = 2 * A[cur] - sum;
            if (t < 0)
            {
                return false;
            }

            long l = A[cur] - t;

            // 子同士通るペアがL個作れるか
            // 子iはc_i回まで使える

            // maxとその他ペア

            // まんべんなくペア
            long r = Math.Min(sum - max, sum / 2);
            if (l > r)
            {
                // Console.WriteLine($"a {cur} {sum} {l} {r} {t}");
                return false;
            }

            return true;
        }
    }

    public static void Main(string[] args) => new Program().Solve();
}

namespace CompLib.Util
{
    using System;
    using System.Linq;

    class Scanner
    {
        private string[] _line;
        private int _index;
        private const char Separator = ' ';

        public Scanner()
        {
            _line = new string[0];
            _index = 0;
        }

        public string Next()
        {
            if (_index >= _line.Length)
            {
                string s;
                do
                {
                    s = Console.ReadLine();
                } while (s.Length == 0);

                _line = s.Split(Separator);
                _index = 0;
            }

            return _line[_index++];
        }

        public string ReadLine()
        {
            _index = _line.Length;
            return Console.ReadLine();
        }

        public int NextInt() => int.Parse(Next());
        public long NextLong() => long.Parse(Next());
        public double NextDouble() => double.Parse(Next());
        public decimal NextDecimal() => decimal.Parse(Next());
        public char NextChar() => Next()[0];
        public char[] NextCharArray() => Next().ToCharArray();

        public string[] Array()
        {
            string s = Console.ReadLine();
            _line = s.Length == 0 ? new string[0] : s.Split(Separator);
            _index = _line.Length;
            return _line;
        }

        public int[] IntArray() => Array().Select(int.Parse).ToArray();
        public long[] LongArray() => Array().Select(long.Parse).ToArray();
        public double[] DoubleArray() => Array().Select(double.Parse).ToArray();
        public decimal[] DecimalArray() => Array().Select(decimal.Parse).ToArray();
    }
}