int lowerBound(vector<int>& a, int x) {
    int l = 0, r = a.size();
    while (l < r) {
        int mid = l + (r - l) / 2;
        if (a[mid] >= x) ________________; // 在此处填入代码
        else l = mid + 1;
    }
    return l;
}