华为Python面试题，排列生成的解法，最优解答

有两个序列a,b, 大小都为n, 序列元素的值为任意整型数，无序；

要求：通过交换a,b中的[序列a元素的和]与[序列b元素的和]之间的差最小。

我的解答：

最后的解答结果都是我见到的测试方法，此方法本质上是穷举法，必然是最优解答。

在递归函数里保存列表is_used(用来保存某个元素是否已经用过了)的时候，一定要注意用copy.deepcopy.

不然保存的所有列表都是[0,0,0,.....], 因为如果不用前叙的方法的话, append()的是所有list的引用，而所有的这些引用指向的是同一个list对象，所以所有的子序列都一样，而这个序列最后返回到了初试状态，都是0，切记切记！

# it can be seen as  a computation problem,# so use un-repeatible enumerate method to find the best resultimport copy# test suit# au: thomas will lee, lzj, # new = [1, 2, 3, 4, 5, 6, 700, 800]  # my 97 91, better than 99new = [93, 91, 90, 82, 81, 74, 74, 74, 74, 68]# store_dict = {min_difference: []}# new = [1, 50, 90, 100, 10000, 10001]  # my 38, 38, worth than 40# new = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]  # 1 , 1 ;1# new = [1, 1, 2, 3, 29, 30, 210, 232, 12311, 12312] # abs(-19)= 19 ,17, better than 21# min_custom_enumerate(new)sub_len = len(new) / 2# to enumerate numbers in the list new# a_iter = [0 for i in xrange(sub_len)]# record whether one element is usedis_used = [0 for i in xrange(len(new))]median = len(new) / 2    # use a list as a stack to store all scenarios    # sf = open('store_file','w')     # min_custom_enumerate_recursive(0)all_list = []def min_custom_enumerate_recursive(k):    if k == sub_len:        all_list.append(copy.deepcopy(is_used))         # min_recode()        # all_list.append(a_iter)        # sf.write(str(is_used))        # all_list += is_used    else:        for i in xrange(len(new)):            if not is_used[i]:                # all_list.append(is_used)                # a_iter[k] = new[i]                is_used[i] = 1  # used                min_custom_enumerate_recursive(k + 1)                is_used[i] = 0  # after the recursion backtracksdef solve():        # len(new) is always even    min_custom_enumerate_recursive(0)            # with open("store_file") as ff:    # # c = 1    #      str(ff.readlines())    # print all_list    min_difference = abs(sum(new[:median]) - sum(new[median:]))    # min_difference = 1000000000000    all_list_len = len(all_list)    final_a = []    final_b = []    for line in all_list:        line_a = [new[i] for i in xrange(len(line)) if line[i] == 1]        line_b = [new[i] for i in xrange(len(line)) if line[i] == 0]        # print 'line_a',line_a        # print 'line_b',line_b        # print line_b        line_a_sum = sum(line_a)        line_b_sum = sum(line_b)        new_difference = abs(line_a_sum - line_b_sum)        if new_difference < min_difference:            min_difference = new_difference            final_a = copy.deepcopy(line_a)            final_b = copy.deepcopy(line_b)    print 'smallest value is', min_difference    print 'a', final_a    print 'b', final_bsolve()