107. Binary Tree Level Order Traversal II（二叉树的层次遍历 II）

题目链接：https://leetcode.com/problems/binary-tree-level-order-traversal-ii/

思路很简单，就是在层次遍历的基础上做了下反转。

AC 1ms Java：

/**
 * 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;
        int level=0;
        Queue<TreeNode> queue=new LinkedList();
        queue.offer(root);
        while(!queue.isEmpty()){
            ans.add(new ArrayList<Integer>());
            int size=queue.size();
            for(int i=0;i<size;i++){
                TreeNode temp=queue.poll();
                ans.get(level).add(temp.val);
                if(temp.left!=null)
                    queue.offer(temp.left);
                if(temp.right!=null)
                    queue.offer(temp.right);
            }
            level++;
        }
        List<List<Integer>> rt=new ArrayList();
        for(int i=ans.size()-1;i>=0;i--){
            rt.add(ans.get(i));
        }
       return rt; 
    } 
}