1137.費氏數列突變版（前三項和）
class Solution(object):
def tribonacci(self, n):
"""
:type n: int
:rtype: int
"""
if n == 0:
return 0
elif n == 1 or n == 2:
return 1
else:
tribonacci_sequence = [0,1,1]
for i in range (3, n+1):
tribonacci_sequence.append (tribonacci_sequence[i-1] + tribonacc
i_sequence[i-2] + tribonacci_sequence[i-3])
return tribonacci_sequence[n]
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