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

/**
 * 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:
    void helper(TreeNode* root, vector<int> &nums)
    {
       if(!root) return;
       helper(root->left, nums);
       nums.push_back(root->val);
       helper(root->right,nums);
    }
    
    bool findTarget(TreeNode* root, int k) {
       
        if(!root) return false;
        vector<int> nums;
        helper(root, nums);
        for(int i=0, j=nums.size()-1; i<j;)
        {
            if((nums[i]+nums[j]) == k) return true;
            ( nums[i]+nums[j]<k )? i++ : j--;
        }
        return false;
    }
};

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