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

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+# 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
+