This is original level order

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class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        level(root, res, 0);
        return res;
    }
    public void level(TreeNode node, List<List<Integer>> res, int depth) {
        if (node == null) return;
        List<Integer> tmp = new LinkedList<>();
        if (depth == res.size()) res.add(tmp);
        res.get(depth).add(node.val);
        level(node.left, res, depth+1);
        level(node.right, res, depth+1);
    }
}

This is zigzag level order:

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class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        level(root, res, 0);
        return res;
    }
    public void level(TreeNode node, List<List<Integer>> res, int depth) {
        if (node == null) return;
        List<Integer> tmp = new LinkedList<>();
        if (depth == res.size()) res.add(tmp);
        if (depth %2 != 0) {
            res.get(depth).add(0, node.val);
        } else {
            res.get(depth).add(node.val);
        }
            level(node.left, res, depth+1);
            level(node.right, res, depth+1);   
        
    }
}

Interesting.

bfs via queue:

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class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        if (root == null) return res;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        boolean flag = false;
        while (!q.isEmpty()) {
            int count = q.size();
            List<Integer> tmp = new LinkedList<>();
            for (int i = 0; i < count; ++i) {
                if (q.peek().left != null) q.offer(q.peek().left);
                if (q.peek().right != null) q.offer(q.peek().right);
                if (flag) {
                    tmp.add(0, q.poll().val);
                } else {
                    tmp.add(q.poll().val);
                }
            }
            res.add(tmp);
            flag = flag ? false : true;
        }
        return res;
    }
}