AcWing 1945. Comparison of two writing methods for cow baseball (two pointers or two points)

double pointer

#include 
#include 
#include 
using namespace std;

const int N = 1010;
int a[N];
int main()
{
    int n;
    cin >> n;

    for(int i = 1; i <= n; i ++) scanf("%d", &a[i]);
    sort(a+1, a+n+1);

    int res = 0;
    for(int i = 1; i <= n-2; i ++)
    {
        for(int j = i+1; j <= n-1; j ++)
        {
            int d = a[j]-a[i];
            int l = j+1, r = j+1;

            while(l <= n && a[l] < a[j]+d) l ++;
            while(r <= n && a[r] <= a[j]+2*d) r ++;
            res += r-l;
        }
    }
    cout << res;
    return 0;
}

two points

#include 
#include 
#include 
using namespace std;

const int N = 1010;
int a[N];
int main()
{
    int n;
    cin >> n;

    for(int i = 1; i <= n; i ++) scanf("%d", &a[i]);
    sort(a+1, a+n+1);

    int ll,rr;
    int res = 0;
    for(int i = 1; i <= n-2; i ++)
    {
        for(int j = i+1; j <= n-1; j ++)
        {
            int d = a[j]-a[i];
            int l = j+1, r = n;

            while(l >1;
                if(a[mid]a[j]+2*d) continue;
            rr = l;

            l = j+1, r = r;
            while(l >1;
                if(a[mid]>=a[j]+d) r=mid;
                else l = mid+1;
            }
            if(a[l]<a[j]+d) continue;
            ll = l;
            res += rr+1-ll;
            //cout << a[i] << " " << a[j] << " " << a[ll]<<"~"<<a[rr]<<endl;
        }
    }
    cout << res;
    return 0;
}

发现对二分的应用还不够熟练,
在求小于某个极大值的右边界以及大于某个极小值的左边界的时候总是把握不好区间的选取