优美的二分模型

int findl(int x) 
{
    int l = 1, r = n + 1;
    while(l < r)
    {
        int mid = l + r >> 1;
        if(a[mid].pos >= x)
            r = mid;
        else
            l = mid + 1;
    }
  //mid取不到r，而r一开始取n+1有利于查找正确答案
}

int findr(int x)                   
{
    int l = 0, r = n;
    while(l < r)
    {
        int mid = l + r + 1 >> 1;
        if(a[mid].pos<= x)
            l = mid;
        else
            r = mid - 1;
    }
   //原理同上
}