一般计算fibonacci的方法：
1 def fibonacci (n): 2 if n == 0 or n == 1: 3 return 1 4 else: 5 return fibonacci(n-1) + fibonacci(n-2)
这样的 call graph for fibonacci with n=4:
当计算 fibonacci(30)的时候还可以，当计算 fibonacci(50)的时候就很慢了。
好的方法是：用dict记录下来之前计算过的值。
A previously computed value that is stored for later use is called a hint.
好的方法是：
previous = {0:1, 1:1}
def fibonacci(n):if previous.has_key(n):return previous[n]else:newValue = fibonacci(n-1) + fibonacci(n-2)previous[n] = newValuereturn newValue 这样就很快了。
类似的计算阶乘：
previous = {1:1,2:2}
def n(i):if previous.has_key(i):return previous[i]else:newValue = i*n(i-1)previous[i] = newValuereturn previous[i] ---How to Think Like a Computer Scientist（Learning with Python）
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不合并在同一索引上)
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