Leetcode107.二叉树的层序遍历II

给定一个二叉树，返回其节点值自底向上的层序遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）

例如：
给定二叉树 [3,9,20,null,null,15,7],

返回其自底向上的层序遍历为：
![YKCS5EH@R0J1QP@}CQFN%M.png

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        LinkedList<List<Integer>> res = new LinkedList<>();
        if(root == null){
            return res;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while(q.size()>0){
            int size = q.size();
            List<Integer> list = new ArrayList<>();
            while(size>0){
                TreeNode cur = q.poll();
                list.add(cur.val);
                if( cur.left != null){
                    q.add(cur.left);
                }
                if(cur.right!= null){
                    q.add(cur.right);
                }
                size--;
            }
            res.addFirst(list);
        }
        return res;
    }
}

notes:实际就是层序遍历，用linkedList.addFirst() api每次层序遍历的往头结点插就可以了