653. Two Sum IV - Input is a BST.py

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

# Example 1:
# Input: 
#     5
#    / \
#   3   6
#  / \   \
# 2   4   7

# Target = 9

# Output: True
# Example 2:
# Input: 
#     5
#    / \
#   3   6
#  / \   \
# 2   4   7

# Target = 28

# Output: False

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# Simple BFS levle-order 
class Solution(object):
    def findTarget(self, root, k):
        """
        :type root: TreeNode
        :type k: int
        :rtype: bool
        """
        if not root:
            return False
        queue = [root]
        s = set()
        while queue:
            node = queue.pop(0)
            # two sum thought
            if k - node.val in s:
                return True
            s.add(node.val)
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)
        return False