【LightOJ 1307 + 二分】

LightOJ 1307

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

#include<cstdio>#include<algorithm>using namespace std;int pa[2011];int main(){    int T,nl = 0,N,i,j,pl,kl,ans;    scanf("%d",&T);    while(T--)    {        scanf("%d",&N);        for(i = 1 ; i <= N ; i++)            scanf("%d",&pa[i]);            sort(pa + 1,pa + 1 + N);            ans = 0;        for(i = 1 ; i <= N -2; i++)            for(j = i + 1 ; j <= N -1; j++)//确定两边            {                pl = pa[i] + pa[j];                kl = lower_bound(pa + j , pa + 1 + N ,pl) - pa; //二分查找第三边                if(kl == j)                    continue;                ans += kl - j - 1;            }            printf("Case %d: %d\n",++nl,ans);    }    return 0;}