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Question

我想定义一个函数flatten(tree) ，使其访问左分支，然后访问，最后访问右分支。

我试过了：

def flatten(tree):
    if is_empty_tree(tree):
        return tree
    else:
        return [left_branch(tree)]+[entry(tree)]+[right_branch(tree)]

但这远不是我想要的输出。

当树是[5, [2, [1, [], []], []], [7, [6, [], []], [10, [], []]]]我是应该得到[1, 2, 3, 5, 6, 7, 10] 1,2,3,5,6,7,10 [1, 2, 3, 5, 6, 7, 10]但是我得到了[[2, [1, [], []], []], 5, [7, [6, [], []], [10, [], []]]]改为[[2, [1, [], []], []], 5, [7, [6, [], []], [10, [], []]]] 。

我如何实现此功能，使其先访问左分支，条目，然后访问右分支并获得我想要的列表？

我定义了以下功能：

def build_tree(entry, left, right):
    return [entry, left, right]

def entry(tree):
    return tree[0]

def left_branch(tree):
    return tree[1]

def right_branch(tree):
    return tree[2]

def is_empty_tree(tree):
    if tree==[]:
        return True
    else:
        return False

Answer 1

这个怎么样：

tree = [5, [2, [1, [], []], []], [7, [6, [], []], [10, [], []]]]

def flatten(tree):
    """Return a flattened list with all tree elements in order.

    tree is a list / tuple with either 0 elements (an empty tree) or at 
    least 3 elements. The first element is the value of the node, the 
    second the left (smaller) branch (another tree), and the
    right the bigger branch.

    E.g:

        tree = [5, [2, [1, [], []], []], [7, [6, [], []], [10, [], []]]]

    returns

        [1, 2, 5, 6, 7, 10]
    """
    if not tree:
        return []
    return flatten(tree[1]) + [tree[0]] + flatten(tree[2])

print flatten(tree)