107.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>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new LinkedList<List<Integer>>();
        if (root == null) return result;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (queue.size() > 0) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            while (size > 0) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
                size--;
            }
            result.add(0, list);
        }
        return result;
    }
}