[leetcode tree]107. Binary Tree Level Order Traversal II

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

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]
方法1：bfs

class Solution(object):
    def levelOrderBottom(self, root):
        res,level,l= [],[[root]],[root]
        if not root:
            return res
        while level and l:
            l = [(node.left,node.right) for node in l]
            l = [v for lr in l for v in lr if v]
            if l:
                level.append(l)
        while level:
            l = level.pop()
            r = [node.val for node in l]
            res.append(r)
        return res

 方法2：bfs

class Solution(object):
    def levelOrderBottom(self, root):
        res,level = [],[root]
        while root and level:
            res.append([node.val for node in level])
            lr = [(node.left,node.right) for node in level]
            level = [node for p in lr for node in p if node]
        res = [node for node in res[::-1]]
        return res

 方法3：dfs，递归

class Solution(object):
    def levelOrderBottom(self, root):
        res = []
        self.dfs(root,0,res)
        return res
        
    def dfs(self,root,level,res):
        if root:
            if len(res)<level+1:
                res.insert(0,[])
            res[-(level+1)].append(root.val)
            self.dfs(root.left,level+1,res)
            self.dfs(root.right,level+1,res)

 方法4：dfs,非递归

class Solution(object):
    def levelOrderBottom(self, root):
        res = []
        stack = [(0,root)]
        while stack:
            level,node = stack.pop()
            if node:
                if len(res) < level+1:
                    res.insert(0,[])
                res[-(level+1)].append(node.val)
                stack.append((level+1,node.right))
                stack.append((level+1,node.left))
        return res