653. Two Sum IV - Input is a BST

Easy

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Given the root of a Binary Search Tree and a target number k, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input: root = [5,3,6,2,4,null,7], k = 9
Output:
 true

Example 2:

Input: root = [5,3,6,2,4,null,7], k = 28
Output:
 false

Constraints:

• The number of nodes in the tree is in the range [1, 104].

• -104 <= Node.val <= 104

• root is guaranteed to be a valid binary search tree.

• -105 <= k <= 105

class Solution:
    def findTarget(self, root: Optional[TreeNode], target: int) -> bool:
        d = defaultdict(int)
        def traverse(root):
            if not root:
                return False
            remain = target - root.val
            if remain in d and d[remain] > 0:
                return True
            d[root.val] += 1 
            return traverse(root.left) or traverse(root.right)
        return traverse(root)