1
/* Definition for a Node.
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 * function Node(val, left, right, next) {
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 *    this.val = val === undefined ? null : val;
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 *    this.left = left === undefined ? null : left;
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 *    this.right = right === undefined ? null : right;
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 *    this.next = next === undefined ? null : next;
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 * };
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 */
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/**
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 * @param {Node} root
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 * @return {Node}
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 */
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var connect = function (root) {
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  if (!root || !root.left || !root.right) {
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    return root;
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  }
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  root.left.next = root.right;
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  if (root.next) {
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    root.right.next = root.next.left;
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  } else {
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    root.right.next = null;
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  }
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  root.left = connect(root.left);
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  root.right = connect(root.right);
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  return root;
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};