leetcode_gym/_problems/binaryTreeZigZagDfs.md

layout, title, last_solved, category, leetcode_url, status

layout	title	last_solved	category	leetcode_url	status
page	Binary Tree Zig Zag (DFS)	2020-07-22	bfs	https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal	Attempted

Problem

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its zigzag level order traversal as:
[
  [3],
  [20,9],
  [15,7]
]
        

Solution

{% highlight python %}

def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:

output = []

def dfs(node, level, output):
    
    if not node:
        return
    
    if len(output) <= level:
        output += [[]]
        
    dfs(node.left, level+1, output)
    dfs(node.right, level+1, output)
    
    if level%2==0:
        output[level].append(node.val)
    else:
        output[level].insert(0, node.val)

dfs(root, 0, output)
return output

{% endhighlight %}