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by Sumit Chourasia | Oct 08, 2020 | Category :coding | Tags : algorithm binary-tree data-structure easy leetcode tree

Two Sum IV - Input is a BST - Tree - Easy - LeetCode

Two Sum IV - Input is a BST - Tree - Easy - LeetCode

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.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {    
    public bool FindTarget(TreeNode root, int k) {
        var set = new HashSet<int>();
        return Helper(root,set,k);
    }
    
    private bool Helper(TreeNode root, HashSet<int> set, int k){
        if(root == null){
            return false;
        }
        
        if(set.Contains(k-root.val)){
            return true;
        }
        
        set.Add(root.val);
        
        return Helper(root.left,set,k)||Helper(root.right,set,k);
    }
    
}

Time Complexity: O(n)

Space Complexity: O(n)