538. 把二叉搜索树转换为累加树

题目链接

function convertBST(root: TreeNode | null): TreeNode | null {
    let lastSum = 0;
    const reverseInorder = (root: TreeNode|null) => {
        if(!root) return null;
        reverseInorder(root.right);
        root.val += lastSum;
        lastSum = root.val;
        reverseInorder(root.left);
    };
    reverseInorder(root);
    return root;
};