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Solution 15: Weaving Levels

Approach Explanation

We use BFS (Level Order Traversal) with a slight modification: we use a boolean flag left_to_right and reverse the level's values whenever the flag is false.

Step-by-Step Logic

  1. If root is None, return [].
  2. Initialize queue with root and left_to_right = True.
  3. While queue is not empty:
    • Get size of current level.
    • Collect level_vals.
    • If not left_to_right, reverse level_vals.
    • Append to result.
    • Toggle left_to_right.
  4. Return result.

Complexity

  • Time Complexity: O(N).
  • Space Complexity: O(W).

Code

from collections import deque

def zigzag_level_order(root):
    if not root:
        return []
        
    res = []
    q = deque([root])
    left_to_right = True
    
    while q:
        level = []
        for _ in range(len(q)):
            node = q.popleft()
            level.append(node.val)
            if node.left: q.append(node.left)
            if node.right: q.append(node.right)
            
        if not left_to_right:
            level.reverse()
        res.append(level)
        left_to_right = not left_to_right
        
    return res