output_description: 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`.
  
  * * *
input_description: 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
attempt: import sys
  sys.setrecursionlimit(200002)
  
  
  def dfs(s):
      global x, G
      for t, d in G[s]:
          if x[t] is None:
              x[t] = x[s] + d
              if not dfs(t):
                  return False
          else:
              if x[t] != x[s] + d:
                  return False
      return True
  
  
  n, m = map(int, input().split())
  x = [None] * n
  G = [[] for _ in range(n)]
  for _ in range(m):
      l, r, d = map(int, input().split())
      G[l-1].append([r-1, d])
      g[r-1].append([l-1, -d])
  res = True
  for i in range(n):
      if x[i] is None:
          x[i] = 0
          res &= dfs(i)
  print('Yes' if res==True, else 'No')
problem_description: 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.
samples: [{"input": "3 3\n    1 2 1\n    2 3 1\n    1 3 2", "output": "Yes\n    \n\nSome possible sets of values (x_1, x_2, x_3) are (0, 1, 2) and (101, 102,\n103).\n\n* * *"}, {"input": "3 3\n    1 2 1\n    2 3 1\n    1 3 5", "output": "No\n    \n\nIf the first two pieces of information are correct, x_3 - x_1 = 2 holds, which\nis contradictory to the last piece of information.\n\n* * *"}, {"input": "4 3\n    2 1 1\n    2 3 5\n    3 4 2", "output": "Yes\n    \n\n* * *"}, {"input": "10 3\n    8 7 100\n    7 9 100\n    9 8 100", "output": "No\n    \n\n* * *"}, {"input": "100 0", "output": "Yes"}]