<p>Given the <code>root</code> of a binary search tree and an integer <code>k</code>, return <code>true</code> <em>if there exist two elements in the BST such that their sum is equal to</em> <code>k</code>, <em>or</em> <code>false</code> <em>otherwise</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/09/21/sum_tree_1.jpg" style="width: 400px; height: 229px;" />
<pre>
<strong>Input:</strong> root = [5,3,6,2,4,null,7], k = 9
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/09/21/sum_tree_2.jpg" style="width: 400px; height: 229px;" />
<pre>
<strong>Input:</strong> root = [5,3,6,2,4,null,7], k = 28
<strong>Output:</strong> false
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li><code>-10<sup>4</sup> &lt;= Node.val &lt;= 10<sup>4</sup></code></li>
	<li><code>root</code> is guaranteed to be a <strong>valid</strong> binary search tree.</li>
	<li><code>-10<sup>5</sup> &lt;= k &lt;= 10<sup>5</sup></code></li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

 class BSTIterator{
    private:
    stack<TreeNode*> st;
    bool isReverse = false;
    public:
    BSTIterator(TreeNode* root,bool reverse){
        isReverse = reverse;
        pushAll(root);
        
    }

    int next(){
        if(isReverse){
           TreeNode* temp =  st.top();
           st.pop();
           if(temp->left)
           pushAll(temp->left);
           return temp->val;
        }
        else{
        TreeNode* temp = st.top();
        st.pop();
        if(temp->right)
        pushAll(temp->right);
        return temp->val;
        }
    }
    void pushAll(TreeNode* root){
        if(isReverse){
            st.push(root);
            while(root->right){
            st.push(root->right);          
            root = root->right;
        }
        }

        else{
        st.push(root);
        while(root->left){
            st.push(root->left);
            root = root->left;
        }
        } 
    }
     
};
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        if(root==NULL){
            return 0;
        }
        BSTIterator r(root,true);

        BSTIterator l(root,false);
        int i = l.next();
        int j = r.next();
        while(i<j){
            cout<<i<<" "<<j<<endl;
            if(i+j==k){
                return true;
            }
            else if(i+j>k){
                j= r.next();
             
            }
            else if(i+j<k){
                  i= l.next();
                
            }
        }

return 0;
        
    }
};
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