19.md

Max Diff between node and ancestor

 int maxAncestorDiff(TreeNode* root) {
        if (!root)
            return 0;
        int minVal = root->val, maxVal = root->val;
        differ(root, minVal, maxVal);
        return diff;
    }

private:
    int diff = 0;

    void differ(TreeNode* root, int minVal, int maxVal) {
        if (!root)
            return;
        diff = max(diff, max(abs(minVal - root->val), abs(maxVal - root->val)));
        minVal = min(minVal, root->val);
        maxVal = max(maxVal, root->val);
        differ(root->left, minVal, maxVal);
        differ(root->right, minVal, maxVal);
    }