class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        visitTree(root);
        return root;
    }
    public void visitTree(TreeNode root){
        if(root != null){
            visitTree(root.right);
            root.val = root.val + sum;
            sum = root.val;
            visitTree(root.left);
        }
    }
}