538.把二叉搜索树转换为累加树

递归实现

  

class Solution {
public:
    int k = 0;
    TreeNode* convertBST(TreeNode* root) {
        if(!root) {
            return nullptr;
        }
        convertBST(root->right);
        k += root->val;
        root->val = k;
        convertBST(root->left);
        return root;
    }
};