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1666. Change the Root of a Binary Tree 👎

  • Time: $O(n)$
  • Space: $O(h)$
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class Solution {
 public:
  Node* flipBinaryTree(Node* root, Node* leaf) {
    return reroot(root, leaf, nullptr);
  }

 private:
  Node* reroot(Node* root, Node* node, Node* newParent) {
    Node* oldParent = node->parent;
    node->parent = newParent;

    // Clean up the child if it's the new parent.
    if (node->left == newParent)
      node->left = nullptr;
    if (node->right == newParent)
      node->right = nullptr;

    // We meet the original root, so we're done.
    if (node == root)
      return node;

    if (node->left)
      node->right = node->left;
    node->left = reroot(root, oldParent, node);

    return node;
  }
};

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class Solution {
  public Node flipBinaryTree(Node root, Node leaf) {
    return reroot(root, leaf, null);
  }

  private Node reroot(Node root, Node node, Node newParent) {
    Node oldParent = node.parent;
    node.parent = newParent;

    // Clean up the child if it's the new parent.
    if (node.left == newParent)
      node.left = null;
    if (node.right == newParent)
      node.right = null;

    // We meet the original root, so we're done.
    if (node == root)
      return node;

    if (node.left != null)
      node.right = node.left;
    node.left = reroot(root, oldParent, node);

    return node;
  }
}