[leetcode] 919. Complete Binary Tree Inserter

Description

A complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible.

Write a data structure CBTInserter that is initialized with a complete binary tree and supports the following operations:

• CBTInserter(TreeNode root) initializes the data structure on a given tree with head node root;
• CBTInserter.insert(int v) will insert a TreeNode into the tree with value node.val = v so that the tree remains complete, and returns the value of the parent of the inserted TreeNode;
• CBTInserter.get_root() will return the head node of the tree.

Example 1:

Input: inputs = ["CBTInserter","insert","get_root"], inputs = [[[1]],[2],[]]
Output: [null,1,[1,2]]

Example 2:

Input: inputs = ["CBTInserter","insert","insert","get_root"], inputs = [[[1,2,3,4,5,6]],[7],[8],[]]
Output: [null,3,4,[1,2,3,4,5,6,7,8]]

Note:

• The initial given tree is complete and contains between 1 and 1000 nodes.
• CBTInserter.insert is called at most 10000 times per test case.
• Every value of a given or inserted node is between 0 and 5000.

分析

题目的意思是：实现一个完全二叉树的插入操作，这道题需要用到队列，先遍历到要插入的位置然后插入就行了，那么可以用队列q1存入插入的位置的父节点。首先，需要找到父结点，利用队列q2，进入队列的顺序是从左到右，找到缺少叶子结点的结点就行了，然后就存入到先前的队列q1。插入的时候也是先插入左叶子结点，如果位置是右结点的话，q1需要把当前的结点移除，表明该结点不需要再插入任何结点了。

代码

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class CBTInserter:

    def __init__(self, root: TreeNode):
        self.root=root
        self.deque=collections.deque()
        q=[self.root]
        while(q):
            node=q.pop(0)
            if(not node.left or not node.right):
                self.deque.append(node)
            if(node.left):
                q.append(node.left)
            if(node.right):
                q.append(node.right)

    def insert(self, v: int) -> int:
        node=self.deque[0]
        self.deque.append(TreeNode(v))
        
        if(node.left is None):
            node.left=self.deque[-1]
        elif(node.right is None):
            node.right=self.deque[-1]
            self.deque.popleft()
        return node.val
           

    def get_root(self) -> TreeNode:
        return self.root
        


# Your CBTInserter object will be instantiated and called as such:
# obj = CBTInserter(root)
# param_1 = obj.insert(v)
# param_2 = obj.get_root()

参考文献

[LeetCode] Python using Level Order Traversal (BFS)