代码随想录：把二叉搜索树转化为累加树

相当于将数组从右到左遍历，下一个数加上一个数，二叉搜索树中序遍历（左中右）为顺序，右中左则为倒叙

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
 * right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* pre = NULL;
    TreeNode* bstToGst(TreeNode* root) {
        if (root == NULL)
            return root;
        if (root->right)
            root->right = bstToGst(root->right);
        if (pre != NULL)
            root->val += pre->val;
        pre = root;
        if (root->left)
            root->left = bstToGst(root->left);
        return root;
    }
};