Skip to content

1214. Two Sum BSTs 👍

  • Time: $O(n)$
  • Space: $O(n)$
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
class BSTIterator {
 public:
  BSTIterator(TreeNode* root, bool leftToRight) : leftToRight(leftToRight) {
    pushUntilNull(root);
  }

  bool hasNext() {
    return !stack.empty();
  }

  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 twoSumBSTs(TreeNode* root1, TreeNode* root2, int target) {
    BSTIterator bst1(root1, true);
    BSTIterator bst2(root2, false);

    for (int l = bst1.next(), r = bst2.next(); true;) {
      const int sum = l + r;
      if (sum == target)
        return true;
      if (sum < target) {
        if (!bst1.hasNext())
          return false;
        l = bst1.next();
      } else {
        if (!bst2.hasNext())
          return false;
        r = bst2.next();
      }
    }
  }
};

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
class BSTIterator {
  BSTIterator(TreeNode root, boolean leftToRight) {
    this.leftToRight = leftToRight;
    pushUntilNull(root);
  }

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

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

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

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

class Solution {
  public boolean twoSumBSTs(TreeNode root1, TreeNode root2, int target) {
    BSTIterator bst1 = new BSTIterator(root1, true);
    BSTIterator bst2 = new BSTIterator(root2, false);

    for (int l = bst1.next(), r = bst2.next(); true;) {
      final int sum = l + r;
      if (sum == target)
        return true;
      if (sum < target) {
        if (!bst1.hasNext())
          return false;
        l = bst1.next();
      } else {
        if (!bst2.hasNext())
          return false;
        r = bst2.next();
      }
    }
  }
}

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
class BSTIterator:
  def __init__(self, root: TreeNode | None, leftToRight: bool):
    self.stack = []
    self.leftToRight = leftToRight
    self._pushUntilNone(root)

  def hasNext(self) -> bool:
    return len(self.stack) > 0

  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: TreeNode | None):
    while root:
      self.stack.append(root)
      root = root.left if self.leftToRight else root.right


class Solution:
  def twoSumBSTs(
      self,
      root1: TreeNode | None,
      root2: TreeNode | None,
      target: int,
  ) -> bool:
    bst1 = BSTIterator(root1, True)
    bst2 = BSTIterator(root2, False)

    l = bst1.next()
    r = bst2.next()
    while True:
      summ = l + r
      if summ == target:
        return True
      if summ < target:
        if not bst1.hasNext():
          return False
        l = bst1.next()
      else:
        if not bst2.hasNext():
          return False
        r = bst2.next()