102. Binary Tree Level Order Traversal

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

    TreeNode(int val) {
        this.val = val;
    }
}

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();

        if (root == null) {
            return result;
        }

        Queue<TreeNode> temp = new LinkedList<>();
        temp.offer(root);

        while (!temp.isEmpty()) {
            int length = temp.size();
            List<Integer> list1 = new ArrayList<>();

            for (int i = 0; i < length; i++) {
                TreeNode node = temp.poll();
                list1.add(node.val);

                if (node.left != null) {
                    temp.offer(node.left);
                }

                if (node.right != null) {
                    temp.offer(node.right);
                }
            }

            result.add(list1);
        }

        return result;
    }
}