102. 二叉树的层序遍历

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var levelOrder = function (root) {
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  if(!root) return []
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  const res = []
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  const queue = [[root, 0]]
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  while(queue.length) {
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    const [ node, level ] = queue.pop()
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    if(res[level]) {
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      res[level].push(node.val)
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    } else {
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      res[level] = [node.val]
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    }
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    if(node.right) queue.push([node.right, level + 1])
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    if(node.left) queue.push([node.left, level + 1])
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  }
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  return res
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};