[LeetCode Plus] Convert BST to Circular DLL

Question

link

Convert a BST to a sorted circular doubly-linked list in-place.

Think of the left and right pointers as synonymous to the previous and next pointers in a doubly-linked list.

One: Inorder

This question can be solved with inorder traversal with the help of a ‘pre’ pointer.

This solution is recommended by L33tcode, but not very intuitive, and difficult to write. The C++ code is attached below. source.

void treeToDoublyList(Node *p, Node *& prev, Node *& head) {
  if (!p) return;
  treeToDoublyList(p->left, prev, head);
  // current node's left points to previous node
  p->left = prev;
  if (prev)
    prev->right = p;  // previous node's right points to current node
  else
    head = p; // current node (smallest element) is head of
              // the list if previous node is not available
  // as soon as the recursion ends, the head's left pointer 
  // points to the last node, and the last node's right pointer
  // points to the head pointer.
  Node *right = p->right;
  head->left = p;
  p->right = head;
  // updates previous node
  prev = p;
  treeToDoublyList(right, prev, head);
}

// Given an ordered binary tree, returns a sorted circular
// doubly-linked list. The conversion is done in-place.
Node* treeToDoublyList(Node *root) {
  Node *prev = NULL;
  Node *head = NULL;
  treeToDoublyList(root, prev, head);
  return head;
}

Two: Divide and conquer

The good and intuitive solution is to do D&C and solve left and right recursively. Do note how the circular linked lists are merged, and be careful when replacing the pointers.

The Java code is posted below. source

public static TreeNode convertBstToDLL(TreeNode root) {
    // convert bst to circular dll 
    if (root == null)
        return (null);

    // Recursively do the subtrees (leap of faith!)
    TreeNode lln = convertBstToDLL(root.left);
    TreeNode rrn = convertBstToDLL(root.right);

    // Make root a circular DLL
    root.left = root;
    root.right = root;

    // At this point we have three lists, merge them
    lln = appendDLL(lln, root);
    lln = appendDLL(lln, rrn);

    return lln;
}

public static TreeNode appendDLL(TreeNode a, TreeNode b) {
    // append 2 circular DLL
    if (a == null)
        return b;
    if (b == null)
        return a;

    TreeNode aLast = a.left;
    TreeNode bLast = b.left;

    aLast.right = b;
    b.left = aLast;
    bLast.right = a;
    a.left = bLast;

    return a;
}