1. Description

Two Sum IV - Input is a BST

2. Solution

Version 1

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        vector<int> numbers;
        inorderTraverse(root, numbers);
        for(int val : numbers) {
            if(k - val == val) {
                return false;
            }
            else if(search(root, k - val)) {
                return true;
            }
        }
        return false;
    }

private:
    void inorderTraverse(TreeNode* root, vector<int>& numbers) {
        if(!root) {
            return;
        }
        inorderTraverse(root->left, numbers);
        numbers.push_back(root->val);
        inorderTraverse(root->right, numbers);
    }

    bool search(TreeNode* root, int target) {
        if(!root) {
            return false;
        }
        if(root->val == target) {
            return true;
        }
        else if(root->val > target) {
            return search(root->left, target);
        }
        else {
            return search(root->right, target);
        }
    }
};

Version 2

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool findTarget(TreeNode* root, int k) {
        return traverse(root, root, k);
    }

private:
    bool traverse(TreeNode* root, TreeNode* head, int k) {
        if(root->left && traverse(root->left, head, k)) {
            return true;
        }
        if(root->right && traverse(root->right, head, k)) {
            return true;
        }
        if(k - root->val == root->val) {
            return false;
        }
        return search(head, k - root->val);
    }

    bool search(TreeNode* root, int target) {
        if(!root) {
            return false;
        }
        if(root->val == target) {
            return true;
        }
        else if(root->val > target) {
            return search(root->left, target);
        }
        else {
            return search(root->right, target);
        }
    }
};