leetcode 107. Binary Tree Level Order Traversal II

正常做bfs，然后List添加的时候，从头部开始添加add(0, ...)

class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        
        Queue<TreeNode> q = new LinkedList<>();
        
        q.offer(root);
        while(!q.isEmpty()){
            int size = q.size();
            List<Integer> level = new ArrayList<>();
            for(int i=0; i<size; i++){
                
                TreeNode cur = q.poll();
                if(cur == null)
                    continue;
                level.add(cur.val);
                q.offer(cur.left);
                q.offer(cur.right);
            }
            if(!level.isEmpty())
                res.add(0, level);
        }
        return res;
    }
}