https://leetcode.com/problems/two-sum-iv-input-is-a-bst/

Leetcode: 653. Two Sum IV - Input is a BST

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func findTarget(root *TreeNode, k int) bool {
    vals := map[int]bool{}
    var helper func(node *TreeNode) bool
    helper = func(node *TreeNode) bool {
        if node == nil {
            return false
        }
        // Note: It's important to check for k-node.Val first, in the case that node.Val == k/2
        if vals[k-node.Val] {
            return true
        }
        vals[node.Val] = true
        return helper(node.Left) || helper(node.Right)
    }
    return helper(root)
}