题意：把二叉查找树每个节点的值都加上比它大的节点的值。
/**
* 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:
int sum = 0;
TreeNode* convertBST(TreeNode* root) {
if(root == NULL) return NULL;
convertBST(root -> right);
sum += root -> val;
root -> val = sum;
convertBST(root -> left);
return root;
}
};