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Project Euler - Problem 14

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  1. #!/usr/bin/python
  2.  
  3. # Project Euler - Problem 14
  4. import math
  5. print "\nProject Euler - Problem 14"
  6. print "For the sequence: \n"
  7. print "n -> n/2 (if n is even)"
  8. print "n -> 3n+1 (if n is odd)\n"
  9. print "Find the longest chain that ends in '1'"
  10.  
  11. start = 10**6
  12. num = (start/2)
  13. ans = 0
  14. maxcnt=0
  15. while num < start:
  16.     x = num
  17.     cnt = 1
  18.     while x != 1:
  19.         if x & 1: x = 3*x + 1 # odd
  20.         else: x >>= 1  # even
  21.         cnt += 1
  22.         if cnt > maxcnt: 
  23.             ans = num
  24.             maxcnt = cnt
  25.     num += 1
  26. print ans

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