23. Two Sum IV - Input is a BST.cpp

/*
Two Sum IV - Input is a BST
===========================

Given the root of a Binary Search Tree and a target number k, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:
Input: root = [5,3,6,2,4,null,7], k = 9
Output: true

Example 2:
Input: root = [5,3,6,2,4,null,7], k = 28
Output: false

Example 3:
Input: root = [2,1,3], k = 4
Output: true

Example 4:
Input: root = [2,1,3], k = 1
Output: false

Example 5:
Input: root = [2,1,3], k = 3
Output: true
 
Constraints:
The number of nodes in the tree is in the range [1, 104].
-104 <= Node.val <= 104
root is guaranteed to be a valid binary search tree.
-105 <= k <= 105
*/

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