/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class Solution {
private:unordered_set<int> s;bool dfs(TreeNode* root,int k, unordered_set<int>& s){if(root==nullptr) return false;if(s.count(k-root->val)) return true;s.insert(root->val);return dfs(root->left,k,s)||dfs(root->right,k,s);}
public:bool findTarget(TreeNode* root, int k) {s.clear();return dfs(root,k,s);}
};

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = Noneclass Solution(object):def helper(self,root,k,s):if not root:return Falseif k-root.val in s:return Trues.add(root.val)return self.helper(root.left,k,s) or self.helper(root.right,k,s)def findTarget(self, root, k):""":type root: TreeNode:type k: int:rtype: bool"""s=set()return self.helper(root,k,s)

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}* };*/
class Solution {
public:string tree2str(TreeNode* t) {if(t==nullptr)return "";string s=to_string(t->val);if(t->left==nullptr){if(t->right==nullptr)return s;else{return s+"()"+"("+tree2str(t->right)+")";}}else{return s+"("+tree2str(t->left)+")"+(!t->right?"":"("+tree2str(t->right)+")");}}
};

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = Noneclass Solution(object):def tree2str(self, t):""":type t: TreeNode:rtype: str"""if not t:return ""if not t.left:return str(t.val)+("()"+"("+self.tree2str(t.right)+")" if t.right else "")else:return str(t.val)+"("+self.tree2str(t.left)+")"+("("+self.tree2str(t.right)+")" if t.right else "")