题目：某个二叉树中是否存在一个元素
输入：二叉树和target
输出：True or False
思路：如果此节点的值==target，则返回True，否则看左右子树中是否存在此元素
代码：
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class :
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def contain(root, key):
if not root:
return False
if root.val == key:
return True
return contain(root.left, key) or contain(root.right, key)
tree = TreeNode(3)
left_tree = TreeNode(9)
right_tree = TreeNode(20)
right_tree.left = TreeNode(15)
right_tree.right = TreeNode(7)
tree.left = left_tree
tree.right = right_tree
print(contain(tree, 200))
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