Lightoj 1307 二分

题目：
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：给你若干根棍子，问能拼出多少多少个三角形。

方法：二分查找。

C++ STL中的upper_bound,lower_bound能减轻不少负担。关于lower_bound,upper_bound的介绍，见百度。

点击打开链接

代码：

//By Sean Chen
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
int t,n,stick[2005];
int cmp(int a,int b)
{
return a<b;
}
int main()
{
scanf("%d",&t);
for (int Case=1;Case<=t;Case++)
{
printf("Case %d: ",Case);
scanf("%d",&n);
for (int i=0;i<n;i++)
scanf("%d",&stick[i]);
sort(stick,stick+n,cmp);
int ans=0;
for (int i=0;i<n-2;i++)
{
for (int j=i+1;j<n-1;j++)
{
int pos=lower_bound(stick+j,stick+n,stick[i]+stick[j])-stick;
if(j!=pos)
ans=ans+(pos-j-1);
}
}
printf("%d\n",ans);
}
return 0;
}