LeetCode - 653. Two Sum IV - Input is a BST

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

Example 1:

Input:
5
/ \
3   6
/ \   \
2   4   7

Target = 9

Output: True

Example 2:

Input:
5
/ \
3   6
/ \   \
2   4   7

Target = 28

Output: False

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean findTarget(TreeNode root, int k) {
if (root == null)
return false;
List<Integer> arr = new ArrayList<Integer>();
toArr(root, arr);
int head = 0, tail = arr.size()-1;
while (head < tail) {
if (arr.get(head)+arr.get(tail) == k)
return true;
else if (arr.get(head)+arr.get(tail) < k) {
head ++;
}
else tail --;
}
return false;
}

private void toArr(TreeNode node, List<Integer> arr) {
if (node == null)
return ;
toArr(node.left, arr);
arr.add(node.val);
toArr(node.right, arr);
}

}