Find a pair with a given sum in BST

main.py

class Solution:
    def findTarget(self, root: TreeNode, k: int) -> bool:
        arr, stack = [], []
        curr_node = root
        
        while curr_node or stack:
            if curr_node:
                stack.append(curr_node)
                curr_node = curr_node.left
            else:
                curr_node = stack.pop()
                arr.append(curr_node.val)
                curr_node = curr_node.right
        
        i, j = 0, len(arr) - 1
        while i < j:
            curr_sum = arr[i] + arr[j]
            if curr_sum == k:
                return True
            if curr_sum < k:
                i += 1
            else:
                j -= 1
        return False