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 Python (2.7.6) (judge verdict: Accepted)

from collections import deque
n, m = map(int, raw_input().split())
graph = {_: set() for _ in xrange(n + 1)}
in_deg, dist = [0] * (n + 1), [-1] * (n + 1)
for _ in xrange(m):
    l, r, d = map(int, raw_input().split())
    graph[l].add((r, d))
    in_deg[r] += 1
que = deque([i for i in xrange(n + 1) if not in_deg[i]])
cst = 1
while que:
    stp = que.popleft()  # start_point
    if in_deg[stp] < 0:
        in_deg[stp] = cst
    for v, d in graph[stp]:
        if dist[v] < 0:
            dist[v] = dist[stp] + d
        else:
            if dist[v] != dist[stp] + d:
                print 'No'
                exit()
        in_deg[v] -= 1
        if not in_deg[v]:
            que.append(v)
print 'No' if [_ for _ in in_deg[1:] if _ > 0] else 'Yes'