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Jun 28th, 2017
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  1. import sys
  2. sys.stdin=open('tree.in','r')
  3. sys.stdout=open('tree.out','w')
  4. n=int(input())
  5. a= [[int(j) for j in input().split()] for i in range(n)]
  6. k1=0
  7. for i in range(n):
  8. for j in range(n):
  9. if a[i][j]==1:
  10. k1+=1
  11.  
  12. visited=[False]*n
  13. def dfs(s):
  14. visited[s]=True
  15. for i in range(n):
  16. if a[s][i]==1 and visited[i]==False:
  17. visited[i]=True
  18. dfs(i)
  19. k=0
  20. l=0
  21. for i in range(n):
  22. if visited[i]==False:
  23. dfs(i)
  24. k+=1
  25.  
  26. if k==1 and k1//2+1==n:
  27. print('YES')
  28. else:
  29. print('NO')
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