Solution32-2.java

/**
 * Definition for a binary tree node. public class TreeNode { int val; TreeNode
 * left; TreeNode right; TreeNode(int x) { val = x; } }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> rowList = new ArrayList<>();

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

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<Integer> colList = new ArrayList<>();
            for (int i = queue.size(); i > 0; i--) {
                TreeNode node = queue.poll();
                colList.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            rowList.add(colList);
        }

        return rowList;

    }
}