C. A Tale of Two Lands

链接：http://codeforces.com/contest/1166/problem/C

题意：问有多少组（俩俩一组）组合，能使|a|，|b|在|a+b|和|a-b|之间

题解：主要是要理解性质，同时思考如何节省时间。

代码：

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<vector>
#include<stack> 
#include<cstring>
#include<map>
#include<queue> 
#include<cmath>
#include<set> 
#include<deque>
#define INF 0x3f3f3f3f
#define lowbit(a) ((a)&-(a))
#define speed std::ios::sync_with_stdio(false),cin.tie(NULL),cout.tie(NULL)
using namespace std;
typedef long long ll;
queue<ll> q;
stack<ll> s;
deque<ll> deq;
priority_queue<ll> pq;  
const ll maxn = 200005;
ll n,m,l,r;
ll ans=0;
ll a[maxn]; 
int main()
{
 cin>>n;
 for(ll i=0;i<n;i++){
  cin>>a[i];
  a[i]=abs(a[i]); //只要绝对值 a[r]<=2*a[i] 即可 
 }
 sort(a,a+n);
 r=0;
 for(ll i=0;i<n;i++){
  while((r<n)&&(a[r]<=2*a[i])){ //由于递增的性质，所以r不必从头再开始 
   r++;
  }
  ans+=(r-i-1); //减头减尾 
 }
 cout<<ans<<endl;
}