lightoj-1307-Counting Triangles【思维】

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1307 - Counting Triangles

You are given N sticks having distinct lengths; you have to form some triangles using the sticks. A triangle is valid if its area is positive. Your task is to find the number of ways you can form a valid triangle using the sticks.

Input

Input starts with an integer T (≤ 10), denoting the number of test cases.

Each case starts with a line containing an integer N (3 ≤ N ≤ 2000). The next line contains N integers denoting the lengths of the sticks. You can assume that the lengths are distinct and each length lies in the range [1, 10⁹].

Output

For each case, print the case number and the total number of ways a valid triangle can be formed.

#include<cstdio>
#include<cstring>
#include<algorithm>
#define LL long long
using namespace std;
const int MAXN=5000;
int n;
LL a[MAXN];
int main()
{
  int t,text=0;
  scanf("%d",&t);
  while(t--)
  {
    scanf("%d",&n);
    for(int i=0;i<n;i++)
      scanf("%lld",&a[i]);
    sort(a,a+n);
    LL ans=0;
    for(int i=0;i<n;i++)
    {
      for(int j=i+1;j<n;j++)
      {
        LL b=a[i]+a[j];
        LL pos=lower_bound(a,a+n,b)-a; // 返回第一个 >=b 的位置 
        ans+=pos-1-j;
      }
    }
    printf("Case %d: %lld\n",++text,ans);
  }
  return 0;
}