算法导论2.3-7习题

先使用归并排序进行排序，再建立其补集，对这两个排序后的集合进行元素比较，找到相同元素即寻找到答案

def findthecomplementary(s, x):
    l1 = list(s)
    l2 = []
    n = len(l1)
    mergesort(l1, 0, n-1)
    for a in reversed(l1):
        if (x - a) != a:
            l2.append(x - a)
    n2 = len(l2)
    i, j = 0, 0
    while (i < n) & (j < n2):
        if l1[i] < l2[j]:
            i += 1
        elif l1[i] > l2[j]:
            j += 1
        else:
            print str(l1[i]) + " + " + str(x - l1[i]) + " = " + str(x)
            return
    print "There is no such pair"
    return

s = set([1, 3, 5, 7, 7, 9])
x = 12
findthecomplementary(s, x)