LevelOrderTraversal.java

import java.util.*;

public class LevelOrderTraversal {
    public List<List<Integer>> levelOrder(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        List<List<Integer>> res = new ArrayList<>();
        if (root == null)
            return res;
        q.add(root);
        while (!q.isEmpty()) {
            int size = q.size();
            List<Integer> level = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode curr = q.poll();
                if (curr.left != null)
                    q.add(curr.left);
                if (curr.right != null)
                    q.add(curr.right);
                level.add(curr.val);
            }
            res.add(level);
        }
        return res;
    }

    public List<Integer> levelOrder1(TreeNode root) {
        Queue<TreeNode> q = new LinkedList<>();
        List<Integer> res = new ArrayList<>();
        if (root == null)
            return res;
        q.offer(root);
        while (!q.isEmpty()) {
            int size = q.size();
            for (int i = 0; i < size; i++) {
                if (q.peek().left != null)
                    q.offer(q.peek().left);
                if (q.peek().right != null)
                    q.offer(q.peek().right);
                res.add(q.poll().val);
            }
        }
        return res;
    }
}