Solution32-1.java

/**
 * Definition for a binary tree node. public class TreeNode { int val; TreeNode
 * left; TreeNode right; TreeNode(int x) { val = x; } }
 */
class Solution {
    public int[] levelOrder(TreeNode root) {
        if (root == null) {
            return new int[0];
        }

        List<Integer> resultList = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            resultList.add(node.val);

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

        int[] result = new int[resultList.size()];
        for (int i = 0; i < result.length; i++) {
            result[i] = resultList.get(i);
        }

        return result;

    }
}