#!/anaconda/bin/python
# coding: utf-8

def for_sum(l):
    sum = 0
    for i in l:
        sum += i
    return sum

def while_sum(l):
    sum = 0
    while len(l) > 0:
        sum += l.pop()
    return sum

def recurr_sum(l):
    if len(l) == 0:
        return 0
    elif len(l) == 1:
        return l[0]
    else:
        return l[0] + recurr_sum(l[1:])

def concat(l1, l2):
    ret = []
    while len(l1) > 0 or len(l2) > 0:
        ret.append(l2.pop())
        ret.append(l1.pop())
    ret.reverse()
    return ret

def fib(n):
    ret = []
    ret.append(0)
    ret.append(1)
    for i in range(2, n):
        ret.append(ret[i-1] + ret[i-2])
    return ret[n-1]

def max_digit(l):
    max = -1
    import itertools
    perms = itertools.permutations(l)
    for p in perms:
        s = ''
        for d in p:
            s += str(d)
        if max < int(s):
            max = int(s)
    return max

def eval_100():
    import itertools
    ops = "+- "
    iter = itertools.product('1', ops, '2', ops, '3', ops, '4', ops,
                             '5', ops, '6', ops, '7', ops, '8', ops, '9')
    for i in iter:
        ret = eval(''.join(i).replace(' ', ''))
        if ret == 100:
            print(''.join(i).replace(' ', ''))

if __name__ == "__main__":
    l = [1, 2, 3, 4, 5]
    print(for_sum(l))
    l = [1, 2, 3, 4, 5]
    print(while_sum(l))
    l = [1, 2, 3, 4, 5]
    print(recurr_sum(l))

    l1 = [1, 2, 3, 4, 5]
    l2 = ["a", "b", "c", "d", "e"]
    print(concat(l1, l2))

    print(fib(101))

    print(max_digit([50, 2, 1, 9]))
    eval_100()