二分查找
----------------------------------------------------------------------找>=x的第一个位置-----------------------------------------------------------------------------------------------
while(l < r)
{
mid = (l + r) / 2;
if(a[mid] >= x) r = mid;
else l = mid + 1;
}
----------------------------------------------------------------------找<=x的第一个位置-----------------------------------------------------------------------------------------------
while(l < r)
{
mid = (l + r + 1) / 2;
if(a[mid] <= x) l = mid;
else r = mid - 1;
}
二分查找stl
