BSTIterator

/**
 * 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{
  public:
        BSTIterator(TreeNode* root, bool rev=false) : reverse(rev){
            push(root);
        }
        void push(TreeNode* root){
            while(root){
                s.push(root);
                root = (reverse)?root->right:root->left;
            }
        }
        int next(){
            TreeNode* tp =s.top();s.pop();
             push(!reverse ? tp->right : tp->left);
            return tp->val;
        }
private:
    stack<TreeNode*>s;
    bool reverse;
};
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
      BSTIterator leftItr(root),rightItr(root,true);
         int left = leftItr.next(), right = rightItr.next();
        while(left<right){
            if(left+right == k) return true;
            if(left+right>k)
                    right=rightItr.next();
            else
                left=leftItr.next();
        }
        return false;
    }
};