归并排序-Python实现

'''
归并排序时间复杂度：O(nlogn)
'''

def merge(list1, list2):
    arr = []
    len1 = len(list1)
    len2 = len(list2)
    i = 0
    j = 0
    while i < len1 and j < len2:
        if list1[i] > list2[j]:
            arr.append(list2[j])
            j += 1
        else:
            arr.append(list1[i])
            i += 1
    while i < len1:
        arr.append(list1[i])
        i += 1
    while j < len2:
        arr.append(list2[j])
        j += 1
    return arr
def merg_sort(a):
    len_a = len(a)
    if len_a <= 1:
        return a
    list1 = merg_sort(a[:len_a/2])
    list2 = merg_sort(a[len_a/2:])
    return merge(list1, list2)
a = [5,6,7,1,2,3,4] #测试用例
a = merg_sort(a)
print(a)