Binary Tree Level Order Traversal II LeetCode solution in JavaScript

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var levelOrderBottom = function (root) {
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  let solution = [];
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  function dfs(node, level) {
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    if (!node) return null;
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    if (!solution[level]) solution[level] = [];
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    solution[level].push(node.val);
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    dfs(node.left, level + 1);
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    dfs(node.right, level + 1);
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  }
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  dfs(root, 0);
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  return solution.reverse();
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};