给定一个二叉树，返回其节点值自底向上的层次遍历。 （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],

    3
   / \
  9  20
    /  \
   15   7
返回其自底向上的层次遍历为：

[
  [15,7],
  [9,20],
  [3]
]

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrderBottom = function(root) {
    var result = []
    var dfs = (node, level) => {
        if(!node){
            return
        }
        result[level] ? result[level].push(node.val) : result[level] = [node.val];
        level ++;
        dfs(node.left, level);
        dfs(node.right, level);
    }

    dfs(root, 0);
    return result.reverse();
};



/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrderBottom = function(root) {
    var result = [],
        queue = [];
    queue.push(root);

    while(queue.length > 0){
        var len = queue.length;
        var level = [];
        while(len > 0){
            var node = queue.shift();
            level.push(node.val);
            if(node.left){
                queue.push(node.left)
            }
            if(node.right){
                queue.push(node.right)
            }
            len--;
        }
        result.unshift(level);
    }
    return result;
};