653. Two Sum IV - Input is a BST

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 
11 static int wing=[]()
12 {
13     std::ios::sync_with_stdio(false);
14     cin.tie(NULL);
15     return 0;
16 }();
17 
18 class Solution 
19 {
20 public:
21     unordered_set<int> s;
22     bool findTarget(TreeNode* root, int k) 
23     {
24         if(root==NULL)
25             return false;
26         if(s.find(k-root->val)!=s.end())
27             return true;
28         s.insert(root->val);
29         return findTarget(root->left,k)||findTarget(root->right,k);
30     }
31 };

可递归，可迭代，问题不大