1
var connect = function (root) {
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  let curr = root;
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  while (curr != null) {
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    let start = null; // (1)
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    let prev = null;
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    while (curr != null) {
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      // (2)
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      if (start == null) {
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        // (3)
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        if (curr.left) start = curr.left;
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        else if (curr.right) start = curr.right;
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        prev = start; // (4)
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      }
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      if (prev != null) {
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        if (curr.left && prev != curr.left) {
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          prev = prev.next = curr.left; // (5)
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        }
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        if (curr.right && prev != curr.right) {
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          prev = prev.next = curr.right;
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        }
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      }
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      curr = curr.next; // (6)
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    }
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    curr = start; // (7)
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  }
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  return root;
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};