LeetCode-Binary Tree Level Order Traversal II

根据 104 题 Maximum Depth of Binary Tree 第二种使用队列层序遍历树的方法，直接层序遍历，然后逆序输出即可。

/**
 * 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>> lo=new ArrayList<List<Integer>>();
        if(root==null) return lo;
        Queue<TreeNode> q=new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()){
            int size=q.size();
            List<Integer> li=new ArrayList<>();
            for(;size>0;size--){
                TreeNode t=q.poll();
                li.add(t.val);
                if(t.left!=null) q.offer(t.left);
                if(t.right!=null) q.offer(t.right);
            }
            lo.add(li);
        }
        Collections.reverse(lo);
        return lo;
    }
}