653. Two Sum IV - Input is a BST

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

用一個hashset紀錄已經traverse過的數字，在recursive helper function裡如果在hashset裡找得到目標值減去當前節點值的數，就可以直接傳回true。

bool helper(TreeNode* root, unordered_set<int>& st, int k) {
    if (!root) return false;
    if (st.count(k - root->val)) return true;
    st.insert(root->val);
    return helper(root->left, st, k) || helper(root->right, st, k);
}
bool findTarget(TreeNode* root, int k) { // time: O(n); space: O(n)
    unordered_set<int> st;
    return helper(root, st, k);
}