### 107.Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example: Given binary tree [3,9,20,null,null,15,7],

3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

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

### Analyze

/** * 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) {  const printArr = []  if (!root) return printArr  const list = []  list.push({ node: root, level: 0 })  while (list.length > 0) {    const { node, level } = list.shift()    if (!printArr[level]) {      printArr.unshift([])    }    printArr[0].push(node.val)    node.left && (list.push({ node: node.left, level: level + 1 }))    node.right && (list.push({ node: node.right, level: level + 1 }))  }  return printArr}

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