题解 | #二叉树层序遍历 ii#

正常层次遍历步骤，然后把结果reverse一下

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrderBottom (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> temp = new ArrayList<>();

        if(root==null)
            return temp;

        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);

        while(!q.isEmpty()){

            int size = q.size();
            ArrayList<Integer> path = new ArrayList<>();

            while(size > 0 ){
                TreeNode n = q.remove();
                path.add(n.val);

                if(n.left!=null)    q.offer(n.left);
                if(n.right!=null)    q.offer(n.right);

                size--;
            }
            temp.add(new ArrayList<>(path));
        }

        ArrayList<ArrayList<Integer>> res = new ArrayList<>();

        for(int i = temp.size()-1 ; i >= 0 ; i--)
            res.add(temp.get(i));

        return res;
    }
}