LightOJ 1307【暴力 二分】

N - Counting Triangles

Time Limit:2000MS Memory Limit:32768KB 64bit IO Format:%lld & %llu

Description

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, 1e9].

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

Sample Input
3
5
3 12 5 4 9
6
1 2 3 4 5 6
4
100 211 212 121
Sample Output
Case 1: 3
Case 2: 7
Case 3: 4

(刚开始我一看才2000个数以为暴力不会超时，没想到还是超了)

#include<stdio.h>
#include<algorithm>
using namespace std;
int a[2020];
int main()
{
    int T,Case=1;
    scanf("%d",&T);
    while( T-- )
    {
        int n;
        scanf("%d",&n);
        for(int i=0; i<n; i++)
            scanf("%d",&a[i]);
        sort(a,a+n);
        int sum=0;
        for(int i=0; i<n; i++)
        {
            for(int j=i+1; j<n; j++)
            {
                int k=a[i]+a[j];
                int l=lower_bound(a+j,a+n,k)-a;
                if(l==j)
                    continue;
                sum+=l-j-1;
            }
        }
        printf("Case %d: %d\n",Case++,sum);
    }
    return 0;
}