CF1166C A Tale of Two Lands

思路：

搞了半天发现和绝对值无关。

http://codeforces.com/blog/entry/67081

实现：

 #include <bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 int a[];
 int main()
 {
     int n;
     while (cin >> n)
     {
         ll ans = ;
         for (int i = ; i < n; i++) { cin >> a[i]; a[i] = abs(a[i]); }
         sort(a, a + n);
         for (int i = ; i < n; i++)
         {
             ans += upper_bound(a + i + , a + n,  * a[i]) - a - i - ;
         }
         cout << ans << endl;
     }
     return ;
 }

 #include <bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 int a[];
 int main()
 {
     int n;
     while (cin >> n)
     {
         ll ans = ;
         for (int i = ; i < n; i++) { cin >> a[i]; a[i] = abs(a[i]); }
         sort(a, a + n);
         int l = , r = ;
         while (l < n)
         {
             while (r < n && a[r] <=  * a[l]) r++;
             ans += r - l - ;
             l++;
         }
         cout << ans << endl;
     }
     return ;
 }