二分
int l = 0, r = n - 1;
while (l < r)
{
int mid = l + r + 1 >> 1;
if (a[mid] <= x) l = mid;
else r = mid - 1;
}
或者
int l = 0, r = n - 1;
while (l < r)
{
int mid = l + r >> 1;
if (a[mid] <= x) l = mid + 1;
else r = mid;
}