897. Increasing Order Search Tree

问题描述 yield就是 return 返回一个值，并且记住这个返回的位置，下次迭代就从这个位置后(下一行)开始。如果是迭代器则应该使用yield from

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
# 树生成代码
def generate_tree(vals):
    if len(vals) == 0:
        return None
    que = [] # 定义队列
    fill_left = True # 由于无法通过是否为 None 来判断该节点的左儿子是否可以填充，用一个记号判断是否需要填充左节点
    for val in vals:
        node = TreeNode(val) if val is not None else None # 非空值返回节点类，否则返回 None
        if len(que)==0:
            root = node # 队列为空的话，用 root 记录根结点，用来返回
            que.append(node)
        elif fill_left:
            que[0].left = node
            fill_left = False # 填充过左儿子后，改变记号状态
            if node: # 非 None 值才进入队列
                que.append(node)
        else:
            que[0].right = node
            if node:
                que.append(node)
            que.pop(0) # 填充完右儿子，弹出节点
            fill_left = True #
    return root

root = generate_tree([5,3,6,2,4,None,8,1,None,None,None,7,9])

class Solution:
    def increasingBST(self, root: TreeNode) -> TreeNode:
        result = recursive = TreeNode(None)

        def reorder(root):
            if root:
                yield from reorder(root.left)
                yield root.val
                yield from reorder(root.right)
        for value in reorder(root):
            recursive.right = TreeNode(value)
            recursive = recursive.right
        return result.right

运行结果：