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653. Two Sum IV - Input is a BST 👍

  • Time: $O(n)$
  • Space: $O(h) \to O(n)$
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class BSTIterator {
 public:
  BSTIterator(TreeNode* root, bool leftToRight) : leftToRight(leftToRight) {
    pushUntilNull(root);
  }

  int next() {
    TreeNode* root = stack.top();
    stack.pop();
    pushUntilNull(leftToRight ? root->right : root->left);
    return root->val;
  }

 private:
  stack<TreeNode*> stack;
  bool leftToRight;

  void pushUntilNull(TreeNode* root) {
    while (root != nullptr) {
      stack.push(root);
      root = leftToRight ? root->left : root->right;
    }
  }
};

class Solution {
 public:
  bool findTarget(TreeNode* root, int k) {
    if (root == nullptr)
      return false;

    BSTIterator left(root, true);
    BSTIterator right(root, false);

    for (int l = left.next(), r = right.next(); l < r;) {
      const int sum = l + r;
      if (sum == k)
        return true;
      if (sum < k)
        l = left.next();
      else
        r = right.next();
    }

    return false;
  }
};

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class BSTIterator {
  public BSTIterator(TreeNode root, boolean leftToRight) {
    this.leftToRight = leftToRight;
    pushLeftsUntilNull(root);
  }

  public int next() {
    TreeNode root = stack.pop();
    pushLeftsUntilNull(leftToRight ? root.right : root.left);
    return root.val;
  }

  public boolean hasNext() {
    return !stack.isEmpty();
  }

  private Deque<TreeNode> stack = new ArrayDeque<>();
  private boolean leftToRight;

  private void pushLeftsUntilNull(TreeNode root) {
    while (root != null) {
      stack.push(root);
      root = leftToRight ? root.left : root.right;
    }
  }
}

class Solution {
  public boolean findTarget(TreeNode root, int k) {
    if (root == null)
      return false;

    BSTIterator left = new BSTIterator(root, true);
    BSTIterator right = new BSTIterator(root, false);

    for (int l = left.next(), r = right.next(); l < r;) {
      final int sum = l + r;
      if (sum == k)
        return true;
      if (sum < k)
        l = left.next();
      else
        r = right.next();
    }

    return false;
  }
}

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class BSTIterator:
  def __init__(self, root: Optional[TreeNode], leftToRight: bool):
    self.stack = []
    self.leftToRight = leftToRight
    self._pushUntilNone(root)

  def next(self) -> int:
    node = self.stack.pop()
    if self.leftToRight:
      self._pushUntilNone(node.right)
    else:
      self._pushUntilNone(node.left)
    return node.val

  def _pushUntilNone(self, root: Optional[TreeNode]):
    while root:
      self.stack.append(root)
      root = root.left if self.leftToRight else root.right


class Solution:
  def findTarget(self, root: Optional[TreeNode], k: int) -> bool:
    if not root:
      return False

    left = BSTIterator(root, True)
    right = BSTIterator(root, False)

    l = left.next()
    r = right.next()
    while l < r:
      summ = l + r
      if summ == k:
        return True
      if summ < k:
        l = left.next()
      else:
        r = right.next()

    return False