Fibonacci series - Find the fibonacci number of a given integer 'n'

#!/usr/bin/python

import sys

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# Recursive version - revisits the same computation multiple times

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def fibRecurse(n):

    # Some boundary conditions

    if n < 0:

        print('Cannot find Fibonacci of -ve numbers. Returning -1')

        return -1

    if n == 0:

        return 0

    if n == 1 or n == 2:

        return 1

    return (fibRecurse(n - 2) + fibRecurse(n - 1))

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# Iterative version - optimal computation. Works on accumulation

#                     and moving forward

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def fibIterative(n):

    # Some boundary conditions

    if n < 0:

        print('Cannot find Fibonacci of -ve numbers. Returning -1')

        return -1

    if n == 0:

        return 0

    if n == 1 or n == 2:

        return 1

    a = 1 # (n-1)th element

    b = 1 # (n-2)th element

    for i in xrange(3, n+1):

        c = a + b

        b = a

        a = c

    return a

if __name__ == "__main__":

    if len(sys.argv) == 2:

        n = int(sys.argv[1])

    else:

        print 'Usage - requires 1 arguement (n): progName n'

        exit(0)

    result = fibRecurse(n)

    print("fibonacci -recursive of: ", n, " is: ", result)

    result = fibIterative(n)

    print("fibonacci -iterative of: ", n, " is: ", result)