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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.*;
import java.util.*;
import java.util.Map.Entry;

public class Main {
	
	final boolean isDebug = false;
	String fileName = "input.txt";
	FastScanner sc;
	//PrintWriter out;
	final int MOD = (int)1e9+7;
	final int INF = Integer.MAX_VALUE / 2;
	
	/* solve */
	void solve(){
		int N = sc.nextInt();
		int M = sc.nextInt();
		UnionFind uf = new UnionFind(N);
		
		for(int i = 0; i < M; i++){
			int L = sc.nextInt()-1;
			int R = sc.nextInt()-1;
			int D = sc.nextInt();
			if(uf.isSame(L, R)){
				if(uf.diff(L, R) != D){
					System.out.println("No");
					return;
				}
			}else{
				uf.union(L, R, D);
			}
		}
		System.out.println("Yes");
 	}
	
	/* end solve */
	
	/* main */
	public static void main(String[] args) throws IOException {
		new Main().m();
	}
	
	void m() throws IOException {
		long S = System.currentTimeMillis();
		sc = (isDebug) ? new FastScanner(new FileInputStream(fileName)) : new FastScanner(System.in);
		//out = new PrintWriter(System.out);
		solve();
		//out.flush();
		long G = System.currentTimeMillis();
		if(isDebug){
			System.out.println("---Debug---");
			System.out.printf("%8d ms", (G-S));
		}
	}
	/* end main */
}
/* end Main */

class UnionFind{
	int[] par;
	int[] rank;
	int[] size;
	int[] diffWeight;
	int group;
	
	UnionFind(int n){
		par = new int[n];
		rank = new int[n];
		size = new int[n];
		diffWeight = new int[n];
		group = n;
		for(int i = 0; i < n; i++){
			par[i] = i;
			rank[i] = 0;
			size[i] = 1;
			diffWeight[i] = 0;
		}
	}
	
	//経路圧縮しながらrootを求める
	int find(int x){
		if(par[x] == x){
			return x;
		}else{
			int f = find(par[x]);
			diffWeight[x] += diffWeight[par[x]];
			return par[x] = f;
		}
	}
	
	int weight(int x){
		find(x);
		return diffWeight[x];
	}
	
	//xのほうがrootに近ければ(ポテンシャルが低ければ)値は正になる
	int diff(int x, int y){
		return weight(y) - weight(x);
	}
	
	//ランクの小さいほうにつなげる
	boolean union(int x, int y, int w){
		w += weight(x);
		w -= weight(y);
		x = find(x);
		y = find(y);
		if(x == y) return false;
		
		if(rank[x] < rank[y]){
			int tmp = x; x = y; y = tmp;
			w = -w;
		}
		
		//y(のroot)をx(のroot)の下にくっつける
		//yのほうが重くなる(ポテンシャルが高くなる)
		if(rank[x] == rank[y]) rank[x]++;
		group--;
		par[y] = x;
		size[x] += size[y];
		diffWeight[y] = w;
		
		return true;
	}
	
	boolean union(int x, int y){
		return union(x, y, 0);
	}
	
	boolean isSame(int x, int y){
		return find(x) == find(y);
	}
}

class FastScanner {
    private InputStream in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    public FastScanner(InputStream in) {
		this.in = in;
	}
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() {
    	if (hasNextByte()) return buffer[ptr++];
    	else return -1;
    }
    private static boolean isPrintableChar(int c){
    	return 33 <= c && c <= 126;
    }
    public boolean hasNext() {
    	while(hasNextByte() && !isPrintableChar(buffer[ptr]))
    		ptr++; return hasNextByte();
    }
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() {
    	return Double.parseDouble(next());
    }
}