原题链接: https://oj.leetcode.com/problems/binary-tree-level-order-traversal-ii/

public List<List<Integer>> levelOrderBottom(TreeNode root) {
    LinkedList<List<Integer>> result = new LinkedList<List<Integer>>();

    if (root == null) return result;

    ArrayDeque<TreeNode> queue = new ArrayDeque<TreeNode>();

    queue.offer(root);

    int lastNum = 1;
    int curNum = 0;
    List<Integer> row = new LinkedList<Integer>();

    while (!queue.isEmpty()) {
        lastNum--;

        TreeNode node = queue.poll();
        row.add(node.val);

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

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

        if (lastNum == 0) {
            result.addFirst(row);

            lastNum = curNum;
            curNum = 0;
            row = new LinkedList<Integer>();
        }
    }

    return result;
}