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：

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 int a[200005];
 5 int main()
 6 {
 7     int n;
 8     while (cin >> n)
 9     {
10         ll ans = 0;
11         for (int i = 0; i < n; i++) { cin >> a[i]; a[i] = abs(a[i]); }
12         sort(a, a + n);
13         for (int i = 0; i < n; i++)
14         {
15             ans += upper_bound(a + i + 1, a + n, 2 * a[i]) - a - i - 1;
16         }
17         cout << ans << endl;
18     }
19     return 0;
20 }

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 int a[200005];
 5 int main()
 6 {
 7     int n;
 8     while (cin >> n)
 9     {
10         ll ans = 0;
11         for (int i = 0; i < n; i++) { cin >> a[i]; a[i] = abs(a[i]); }
12         sort(a, a + n);
13         int l = 0, r = 0;
14         while (l < n)
15         {
16             while (r < n && a[r] <= 2 * a[l]) r++;
17             ans += r - l - 1;
18             l++;
19         }
20         cout << ans << endl;
21     }
22     return 0;
23 }