653-two-sum-iv-input-is-a-bst

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Description

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

Solution(Python)

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def findTarget(self, root: Optional[TreeNode], k: int) -> bool:
        def pushLeft(st, root):
            while root:
                st.append(root)
                root = root.left

        def pushRight(st, root):
            while root:
                st.append(root)
                root = root.right

        def nextLeft(st):
            node = st.pop()
            pushLeft(st, node.right)
            return node.val

        def nextRight(st):
            node = st.pop()
            pushRight(st, node.left)
            return node.val

        stLeft, stRight = [], []
        pushLeft(stLeft, root)
        pushRight(stRight, root)

        left, right = nextLeft(stLeft), nextRight(stRight)
        while left < right:
            if left + right == k: return True
            if left + right < k:
                left = nextLeft(stLeft)
            else:
                right = nextRight(stRight)
        return False