Given a binary tree, return thebottom-up level ordertraversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree[3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

倒过来，就是主要一点，ans.add(0, sub); index then combination applicant

/**
 * 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>> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> sub = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode head = queue.poll();
                if (head != null) {
                    sub.add(head.val);
                }else{
                    continue;
                }
                if (head.left != null) {
                    queue.offer(head.left);
                }
                if (head.right != null) {
                    queue.offer(head.right);
                }
            }
            ans.add(0, sub);
        }
        return ans; 
    }