lightoj 1307 Counting Triangles （二分）

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, 10⁹].

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

题意：输入n个长度，问能构成的三角形的个数。

用二分查找来写。

#include<cstdio>#include<algorithm>using namespace std;long long a[2005]; int main(){int t,flag=1;scanf("%d",&t);while(t--){int n;scanf("%d",&n);for(int i=0;i<n;i++){scanf("%lld",&a[i]);}sort(a,a+n);int ans=0; for(int i=0;i<n-2;i++){for(int j=i+1;j<n-1;j++){long long sum=a[i]+a[j];int p=lower_bound(a+j,a+n,sum)-a;ans+=p-1-j;   //不算p所在位置 }}printf("Case %d: %d\n",flag++,ans);} return 0;}