Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

// BFS iterative
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(!q.isEmpty()) {
            int levelSize = q.size();
            List<Integer> currLevel = new ArrayList<>();
            for (int i = 0; i < levelSize; i++) {
                TreeNode currNode = q.poll();
                currLevel.add(currNode.val);
                if (currNode.left != null) {
                    q.add(currNode.left);
                }
                if (currNode.right != null) {
                    q.add(currNode.right);
                }
            }
            res.add(currLevel);
        }
        return res;
    }
}

// DFS recursive
class Solution2 {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        levelOrderHelper(res, root, 0);
        return res;
    }

    public void levelOrderHelper(List<List<Integer>> res, TreeNode root, int level) {
        if (root == null) {
            return;
        }
        List<Integer> curr;
        if (level >= res.size()) {
            curr = new ArrayList<>();
            curr.add(root.val);
            res.add(curr);
        } else {
            curr = res.get(level);
            curr.add(root.val);
        }
        levelOrderHelper(res, root.left, level + 1);
        levelOrderHelper(res, root.right, level + 1);
    }
}