Problem:
Score : 400 points

Problem Statement

There are N people standing on the x-axis.
Let the coordinate of Person i be x_i.
For every i, x_i is an integer between 0 and 10^9 (inclusive).
It is possible that more than one person is standing at the same coordinate.

You will given M pieces of information regarding the positions of these people.
The i-th piece of information has the form (L_i, R_i, D_i).
This means that Person R_i is to the right of Person L_i by D_i units of distance, that is, x_{R_i} - x_{L_i} = D_i holds.

It turns out that some of these M pieces of information may be incorrect.
Determine if there exists a set of values (x_1, x_2, ..., x_N) that is consistent with the given pieces of information.

Constraints

1 \leq N \leq 100 000

0 \leq M \leq 200 000

1 \leq L_i, R_i \leq N (1 \leq i \leq M)

0 \leq D_i \leq 10 000 (1 \leq i \leq M)

L_i \neq R_i (1 \leq i \leq M)

If i \neq j, then (L_i, R_i) \neq (L_j, R_j) and (L_i, R_i) \neq (R_j, L_j).

D_i are integers.

Input

Input is given from Standard Input in the following format:

N M
L_1 R_1 D_1
L_2 R_2 D_2
:
L_M R_M D_M

Output

If there exists a set of values (x_1, x_2, ..., x_N) that is consistent with all given pieces of information, print Yes; if it does not exist, print No.

Sample Input 1

3 3
1 2 1
2 3 1
1 3 2

Sample Output 1

Yes

Some possible sets of values (x_1, x_2, x_3) are (0, 1, 2) and (101, 102, 103).

Sample Input 2

3 3
1 2 1
2 3 1
1 3 5

Sample Output 2

No

If the first two pieces of information are correct, x_3 - x_1 = 2 holds, which is contradictory to the last piece of information.

Sample Input 3

4 3
2 1 1
2 3 5
3 4 2

Sample Output 3

Yes

Sample Input 4

10 3
8 7 100
7 9 100
9 8 100

Sample Output 4

No

Sample Input 5

100 0

Sample Output 5

Yes

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using Yorukatsu039.Questions;
using Yorukatsu039.Extensions;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace Yorukatsu039.Questions
{
    /// <summary>
    /// ABC087 D
    /// </summary>
    public class QuestionE : AtCoderQuestionBase
    {
        public override IEnumerable<object> Solve(TextReader inputStream)
        {
            var nm = inputStream.ReadIntArray();
            var n = nm[0];
            var m = nm[1];
            var people = new WeightedUnionFindTree(n);

            for (int i = 0; i < m; i++)
            {
                var lrd = inputStream.ReadIntArray();
                var l = lrd[0] - 1;
                var r = lrd[1] - 1;
                var d = lrd[2];
                if (people.IsInSameGroup(l, r))
                {
                    if (people.GetDifferenceBetween(l, r) != d)
                    {
                        yield return "No";
                        yield break;
                    }
                }
                else
                {
                    people.Unite(l, r, d);
                }
            }

            yield return "Yes";
        }

        // See https://kumikomiya.com/competitive-programming-with-c-sharp/
        public class WeightedUnionFindTree
        {
            private WeightedUnionFindNode[] _nodes;
            public int Count => _nodes.Length;
            public int Groups { get; private set; }

            public WeightedUnionFindTree(int count)
            {
                _nodes = Enumerable.Range(0, count).Select(_ => new WeightedUnionFindNode()).ToArray();
                Groups = _nodes.Length;
            }

            public void Unite(int index1, int index2, int weight)
            {
                var succeed = _nodes[index1].Unite(_nodes[index2], weight);
                if (succeed)
                {
                    Groups--;
                }
            }

            public int GetDifferenceBetween(int index1, int index2) => _nodes[index2].Weight - _nodes[index1].Weight;

            public bool IsInSameGroup(int index1, int index2) => _nodes[index1].IsInSameGroup(_nodes[index2]);
            public int GetGroupSizeOf(int index) => _nodes[index].GetGroupSize();

            private class WeightedUnionFindNode
            {
                private int _height;        // rootのときのみ有効
                private int _groupSize;     // 同上
                private WeightedUnionFindNode _parent;
                private int _weight;

                public int Weight
                {
                    get
                    {
                        FindRoot();
                        return _weight;
                    }
                    private set
                    {
                        _weight = value;
                    }
                }

                public WeightedUnionFindNode()
                {
                    _height = 0;
                    _groupSize = 1;
                    _parent = this;
                    Weight = 0;
                }

                public WeightedUnionFindNode FindRoot()
                {
                    if (_parent != this) // not ref equals
                    {
                        var root = _parent.FindRoot();
                        _weight += root._weight;
                        _parent = root;
                    }

                    return _parent;
                }

                public int GetGroupSize() => FindRoot()._groupSize;

                public bool Unite(WeightedUnionFindNode other, int weight)
                {
                    weight += Weight;
                    weight -= other.Weight;

                    var thisRoot = this.FindRoot();
                    var otherRoot = other.FindRoot();

                    if (thisRoot == otherRoot)
                    {
                        return false;
                    }

                    if (thisRoot._height < otherRoot._height)
                    {
                        thisRoot._parent = otherRoot;
                        otherRoot._groupSize += thisRoot._groupSize;
                        otherRoot._height = Math.Max(thisRoot._height + 1, otherRoot._height);
                        thisRoot.Weight = -weight;
                        return true;
                    }
                    else
                    {
                        otherRoot._parent = thisRoot;
                        thisRoot._groupSize += otherRoot._groupSize;
                        thisRoot._height = Math.Max(otherRoot._height + 1, thisRoot._height);
                        otherRoot.Weight = +weight;
                        return true;
                    }
                }

                public bool IsInSameGroup(WeightedUnionFindNode other) => this.FindRoot() == other.FindRoot();

                public override string ToString() => $"{Weight} root:{FindRoot().Weight}";
            }
        }

    }
}


namespace Yorukatsu039
{
    class Program
    {
        static void Main(string[] args)
        {
            IAtCoderQuestion question = new QuestionE();    // 問題に合わせて書き換え
            var answers = question.Solve(Console.In);
            foreach (var answer in answers)
            {
                Console.WriteLine(answer);
            }
        }
    }
}

#region Base Classes

namespace Yorukatsu039.Questions
{

    public interface IAtCoderQuestion
    {
        IEnumerable<object> Solve(string input);
        IEnumerable<object> Solve(TextReader inputStream);
    }

    public abstract class AtCoderQuestionBase : IAtCoderQuestion
    {
        public IEnumerable<object> Solve(string input)
        {
            var stream = new MemoryStream(Encoding.Unicode.GetBytes(input));
            var reader = new StreamReader(stream, Encoding.Unicode);

            return Solve(reader);
        }

        public abstract IEnumerable<object> Solve(TextReader inputStream);
    }

}

#endregion

#region Extensions

namespace Yorukatsu039.Extensions
{
    internal static class TextReaderExtensions
    {
        internal static int ReadInt(this TextReader reader) => int.Parse(ReadString(reader));
        internal static long ReadLong(this TextReader reader) => long.Parse(ReadString(reader));
        internal static double ReadDouble(this TextReader reader) => double.Parse(ReadString(reader));
        internal static string ReadString(this TextReader reader) => reader.ReadLine();

        internal static int[] ReadIntArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(int.Parse).ToArray();
        internal static long[] ReadLongArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(long.Parse).ToArray();
        internal static double[] ReadDoubleArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(double.Parse).ToArray();
        internal static string[] ReadStringArray(this TextReader reader, char separator = ' ') => reader.ReadLine().Split(separator);
    }
}

#endregion