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Binary Tree Zigzag Level Order Traversal

medium
Acceptance: 46%
Binary Tree

Traverse a binary tree level by level, but alternate the reading direction at each depth: left-to-right for the first level, right-to-left for the second, left-to-right for the third, and so on. Return the values grouped by level.

Examples

Example 1:
Input:root = [1,2,3,4,5,6,7]
Output:[[1],[3,2],[4,5,6,7]]
Example 2:
Input:root = [9]
Output:[[9]]

Hints

Related Problems

Maximum Depth of Binary TreeeasyInvert Binary TreeeasyValidate Binary Search TreemediumLowest Common Ancestor of a Binary TreemediumBinary Tree Level Order Traversalmedium
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import java.util.*;

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean leftToRight = true;
        while (!queue.isEmpty()) {
            int size = queue.size();
            LinkedList<Integer> level = new LinkedList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (leftToRight) level.addLast(node.val);
                else level.addFirst(node.val);
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            result.add(level);
            leftToRight = !leftToRight;
        }
        return result;
    }
}
Time complexityO(n)
Space complexityO(n)