二叉树的锯齿形层序遍历

// Solution using Java Stream (though not the typical use case for streams)
// The stream API is not well-suited for this problem due to its stateful nature.
// We will use traditional methods but demonstrate where streams might be applied.
import java.util.*;
import java.util.stream.Collectors;
 
public class ZigzagLevelOrderStream {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
 
        Deque<TreeNode> deque = new LinkedList<>();
        deque.offer(root);
        boolean leftToRight = true;
 
        while (!deque.isEmpty()) {
            int size = deque.size();
            List<Integer> level = new ArrayList<>(size);
            for (int i = 0; i < size; i++) {
                TreeNode node = deque.poll();
                level.add(node.val);
                if (node.left != null) deque.offer(node.left);
                if (node.right != null) deque.offer(node.right);
            }
            if (!leftToRight) {
                Collections.reverse(level); // Reverse the list if needed
            }
            result.add(level);
            leftToRight = !leftToRight;
        }
        return result;
    }
}
 
// TreeNode class definition for the binary tree structure
class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}