Binary Tree Level Order Traversal

Binary Tree Level Order Traversal

问题：

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

思路：

　　bfs

我的代码：

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> rst = new ArrayList<List<Integer>>();
        if(root == null) return rst;
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty())
        {
            int size = queue.size();
            List<Integer> list = new ArrayList<Integer>();
            for(int i = 0; i < size; i++)
            {
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left != null)
                    queue.offer(node.left);
                if(node.right != null)
                    queue.offer(node.right);
            }
            rst.add(list);
        }
        // Collections.reverse(rst);
        return rst;
    }
}