binary-tree-level-order-traversal-ii.py

"""

107. Binary Tree Level Order Traversal II
Medium


Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).

 

Example 1:


Input: root = [3,9,20,null,null,15,7]
Output: [[15,7],[9,20],[3]]
Example 2:

Input: root = [1]
Output: [[1]]
Example 3:

Input: root = []
Output: []
 

Constraints:

The number of nodes in the tree is in the range [0, 2000].
-1000 <= Node.val <= 1000

"""

# V0
class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        self.level(root, 0, res)
        return res[::-1]

    def level(self, root, level, res):
        if not root: return
        if len(res) == level: res.append([])
        res[level].append(root.val)
        if root.left: self.level(root.left, level + 1, res)
        if root.right: self.level(root.right, level + 1, res)

# V0'
# IDEA : BFS 
# https://github.com/yennanliu/CS_basics/blob/master/leetcode_python/Breadth-First-Search/binary-tree-level-order-traversal.py
class Solution(object):
    def levelOrderBottom(self, root):
        res = []
        if not root: return res
        queue = collections.deque()
        queue.append(root)
        while queue:
            level = []
            for i in range(len(queue)):  # NOTE THAT HERE WE HAVE TO GO THROUGH EVERY ELEMENT IN THE SAME LAYER OF BST 
                node = queue.popleft()
                level.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            res.append(level)
        return res[::-1]

# V1 
# https://blog.csdn.net/fuxuemingzhu/article/details/49108449
# IDEA : DFS 
class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        self.level(root, 0, res)
        return res[::-1]

    def level(self, root, level, res):
        if not root: return
        if len(res) == level: res.append([])
        res[level].append(root.val)
        if root.left: self.level(root.left, level + 1, res)
        if root.right: self.level(root.right, level + 1, res)
        
# V1' 
# https://blog.csdn.net/fuxuemingzhu/article/details/49108449
# IDEA : BFS  
class Solution:
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        que = collections.deque()
        que.append(root)
        level = 0
        while que:
            levelVal = []
            size = len(que)
            for _ in range(size):
                node = que.popleft()
                if not node:
                    continue
                levelVal.append(node.val)
                que.append(node.left)
                que.append(node.right)
            level += 1
            if levelVal:
                res.append(levelVal)
        return res[::-1]

# V2 
# Time:  O(n)
# Space: O(n)
class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root is None:
            return []

        result, current = [], [root]
        while current:
            next_level, vals = [], []
            for node in current:
                vals.append(node.val)
                if node.left:
                    next_level.append(node.left)
                if node.right:
                    next_level.append(node.right)
            current = next_level
            result.append(vals)

        return result[::-1]