//整数二分
 while (l < r)
{
    int mid = l + r >> 1;
    if (q[mid] >= x) r = mid;//中间点变为右边界
    else l = mid + 1;
}
while (l < r)
{
    int mid = l + r + 1 >> 1;
    if (q[mid] <= x) l = mid;//中间点变为左边界
    else r = mid - 1;
}
//浮点数二分将判定条件中改为r - l > 1e-8