0107-binary-tree-level-order-traversal-ii.java

//Same just reverse in the end.
//Reversing in the end is better than using add(0, E) in case of arraylist as it's an O(1) operation.

class Solution {

    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if (root == null) return ans;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while (!q.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            int size = q.size();
            for (int i = 0; i < size; i++) {
                TreeNode cur = q.poll();
                level.add(cur.val);
                if (cur.left != null) q.offer(cur.left);
                if (cur.right != null) q.offer(cur.right);
            }
            ans.add(level);
        }
        int i = 0, j = ans.size() - 1;
        //reverse the list
        Collections.reverse(ans);
        return ans;
    }
}