103. Binary Tree Zigzag Level Order Traversal

Problem Statement

link: https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/ https://leetcode.cn/problems/binary-tree-zigzag-level-order-traversal/

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

Example:

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

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

Input: root = [] Output: []

Solution Approach

The solution employs depth-first search (DFS) to traverse the tree and utilizes a deque to maintain the order of node values based on the current level of traversal.

Algorithm

1. Using a recursive dfs function, traverse the tree nodes based on their depth level.
2. Depending on the current level, either append or prepend the node’s value to the respective deque to achieve the zigzag order.
3. Convert the list of deques into a list of lists after traversal to obtain the final zigzag level order result.

Implement

    class Solution:
      def zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
          res = []
          def dfs(node,level):
              if not node:
                  return
              # add node.val in level
              if level < len(res):
                  if level % 2:
                      res[level].appendleft(node.val)
                  else:
                      res[level].append(node.val)
              else:
                  res.append(deque([node.val]))

              if node.left:
                  dfs(node.left, level + 1)
              if node.right:
                  dfs(node.right, level + 1)
          dfs(root, 0)
          return [list(d) for d in res]
                       
 

    
    

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