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117. Populating Next Right Pointers in Each Node II 👍

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  Node* connect(Node* root) {
    Node* node = root;  // the node that is above the current needling

    while (node) {
      Node dummy(0);  // the dummy node before needling
      // Needle the children of the node.
      for (Node* needle = &dummy; node; node = node->next) {
        if (node->left) {  // Needle the left child.
          needle->next = node->left;
          needle = needle->next;
        }
        if (node->right) {  // Needle the right child.
          needle->next = node->right;
          needle = needle->next;
        }
      }
      node = dummy.next;  // Move the node to the next level.
    }

    return root;
  }
};

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class Solution {
  public Node connect(Node root) {
    Node node = root; // the node that is above the current needling

    while (node != null) {
      Node dummy = new Node(); // a dummy node before needling
                               // Needle the children of the node.
      for (Node needle = dummy; node != null; node = node.next) {
        if (node.left != null) { // Needle the left child.
          needle.next = node.left;
          needle = needle.next;
        }
        if (node.right != null) { // Needle the right child.
          needle.next = node.right;
          needle = needle.next;
        }
      }
      node = dummy.next; // Move the node to the next level.
    }

    return root;
  }
}

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class Solution:
  def connect(self, root: 'Node') -> 'Node':
    node = root  # the node that is above the current needling

    while node:
      dummy = Node(0)  # a dummy node before needling
      # Needle the children of the node.
      needle = dummy
      while node:
        if node.left:  # Needle the left child.
          needle.next = node.left
          needle = needle.next
        if node.right:  # Needle the right child.
          needle.next = node.right
          needle = needle.next
        node = node.next
      node = dummy.next  # Move the node to the next level.

    return root