93. Binary Tree Zigzag Level Order Traversal

Do a normal level-order (BFS) traversal, but reverse the order of values on every other level so the output snakes left-to-right, then right-to-left.

Idea

Zigzag order is just level-order with a twist: even levels read left-to-right, odd levels right-to-left. So we do a standard breadth-first traversal collecting each level's values, and simply reverse the collected list on alternating levels. Nodes are always enqueued left child then right child — only the output order per level flips.

Algorithm

1. BFS with a queue; keep a boolean left_to_right starting True.
2. For each level, pop all count nodes, collect their values (always enqueue children left-then-right).
3. If left_to_right is False, reverse the level's list before appending.
4. Flip left_to_right and continue.

Worked example: [3,9,20,null,null,15,7]

• Level 0 (L→R): [3].
• Level 1 (R→L): collected [9,20], reversed → [20,9].
• Level 2 (L→R): [15,7].

Result: [[3],[20,9],[15,7]].

Complexity

• Time: O(n) — each node visited once.
• Space: O(n) for the queue and output.

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        boolean leftToRight = true;
        while (!q.isEmpty()) {
            int count = q.size();
            List<Integer> level = new ArrayList<>();
            for (int i = 0; i < count; i++) {
                TreeNode node = q.poll();
                level.add(node.val);
                if (node.left != null) q.offer(node.left);
                if (node.right != null) q.offer(node.right);
            }
            if (!leftToRight) {
                Collections.reverse(level);
            }
            result.add(level);
            leftToRight = !leftToRight;
        }
        return result;
    }
}