<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 Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        stack<TreeNode*>st1,st2;
        TreeNode *ptr1=root;
        TreeNode *ptr2 =root;
        while(true)
        {
               while(ptr1!=nullptr)
               {   st1.push(ptr1);
                   ptr1=ptr1->left;
               }

               while(ptr2!=nullptr)
               {
                  st2.push(ptr2);
                  ptr2=ptr2->right;
               }

               if(st1.empty()||st2.empty())
               return false;

               if(st1.top()->val>st2.top()->val)
               return false;
               if(st1.top()==st2.top())
               return false;

               if(st1.top()->val+st2.top()->val==k)
               return true;
               else if(st1.top()->val+st2.top()->val<k)
               {
                   ptr1 = st1.top()->right;
                   st1.pop();
                   
               }
               else
               {
                   ptr2 = st2.top()->left;
                   st2.pop();
               }
        }

        return false;


    }
};
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