Leetcode653.Two Sum IV - Input is a BST两数之和4-输入BST

给定一个二叉搜索树和一个目标结果，如果 BST 中存在两个元素且它们的和等于给定的目标结果，则返回 true。

struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};

class Solution {
public:
    TreeNode* node;
    bool findTarget(TreeNode* root, int k) {
        if(root == NULL)
            return false;
        node = root;
        return GetAns(root, k);
    }

    bool GetAns(TreeNode* root, int k)
    {
        if(root == NULL)
            return false;
        int temp = k - root ->val;
        TreeNode* t = Search(node, temp);
        if(t != NULL && t != root)
            return true;
        return GetAns(root ->left, k) || GetAns(root ->right, k);
    }

    TreeNode* Search(TreeNode* root, int k)
    {
        if(root == NULL)
            return NULL;
        if(root ->val == k)
            return root;
        else if(root ->val > k)
            return Search(root ->left, k);
        else return Search(root ->right, k);
    }
};