Suppose we have two piles of cards face up on a table. Each pile is sorted, with the smallest cards on top. We wish to merge the two piles into a single sorted output pile, which is to be face down on th table. Our basic step consists of choosing the smaller of the two cards on top of the face-up piles, removing it from its pile(which exposes a new top card), and placing this card face down onto the output pile. We repeat this step until one input pile is empty.

假設有兩堆已經排好的牌，現在我們要將他們合併成一堆，那麼只需要在每一堆設定一個標兵，那個標兵表示的數小就把這個數存到我們所需的數組裡面，之後標兵會後退一步，直到整個迭代結束。

這裡首要注意的是，下面這兩句話，此處不解釋，只有單步除錯過才會明白為什麼要這麼寫：

left.append(float("inf")) # 陣列尾以正無窮結束

right.append(float("inf")) # 陣列尾以正無窮結束

# 歸併排序
def merge(arr, p, q, r):
    left = []
    right = []
    for i in range(p, q + 1):
        left.append(arr[i])
    for j in range(q + 1, r + 1):
        right.append(arr[j])
    left.append(float("inf"))
    right.append(float("inf"))
    i = 0
    j = 0
    for k in range(p, r + 1):
            if left[i] <= right[j]:
                arr[k] = left[i]
                i = i + 1
            else:
                arr[k] = right[j]
                j = j + 1


def merge_sort(arr, p, r):
    if p < r:
        q = (p + r) // 2
        merge_sort(arr, p, q)
        merge_sort(arr, q + 1, r)
        merge(arr, p, q, r)


array = [4, 3, 2, -7, -1, 6, 3, 10]
merge_sort(array, 0, len(array) - 1)
print(array)