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107. 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）

链接

function levelOrderBottom(root: TreeNode | null): number[][] {
  if (root === null) return []
  const nodeList: Array<TreeNode | null> = [root]
  // let isResverse = false
  const res = []
  while(nodeList.length > 0) {
    // const method = isResverse ? Array.prototype.unshift : Array.prototype.push
    // isResverse = !isResverse
    if (nodeList.length === 1) {
      const parent = nodeList.shift()
      nodeList.push(parent.left)
      nodeList.push(parent.right)
      res.unshift([parent.val])
    } else {
      const nodeL = nodeList.splice(0)
      const len = nodeL.length
      const valList = []
      for(let i = 0; i < len;i++) {
        const node = nodeL[i]
        if (node) {
          valList.push(node.val)
          nodeList.push(node.left)
          nodeList.push(node.right)
        }
      }
      valList.length > 0 && res.unshift(valList)
    }
  }
  return res
}