给你二叉树的根节点 root ，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）

示例 1：

输入：root = [3,9,20,null,null,15,7]
输出：[[15,7],[9,20],[3]]

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[][]}
 */
var levelOrderBottom = function(root) {
    if(root === null) return []
    let queue = []
    let ans = []
    queue.push([root])

    while (queue.length) {
        let curr = queue.shift()
        if (curr.length) {
            let curAns = []
            let prev = []
            for (let i = 0; i < curr.length; i++) {
                curAns.push(curr[i].val)

                if (curr[i].left) {
                    prev.push(curr[i].left)
                }
                if (curr[i].right) {
                    prev.push(curr[i].right)
                }
            }
            ans.unshift(curAns)
            queue.push(prev)
        }
    }
    return ans
};

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/