poj3109 Inner Vertices

思路：

树状数组 + 扫描线。

实现：

 #include <cstdio>
 #include <vector>
 #include <algorithm>
 using namespace std;
 const int MAXN = ;
 int x[MAXN], y[MAXN], bit[MAXN], cnt[MAXN], now[MAXN], n;
 bool vis[MAXN];
 vector<int> lines[MAXN];
 int lowbit(int x) { return x & -x; }
 void add(int i, int x)
 {
     while (i <= MAXN) { bit[i] += x; i += lowbit(i); }
 }
 int sum(int i)
 {
     int ans = ;
     while (i) { ans += bit[i]; i -= lowbit(i); }
     return ans;
 }
 int compress(int * a)
 {
     vector<int> v(a, a + n);
     sort(v.begin(), v.end());
     v.erase(unique(v.begin(), v.end()), v.end());
     for (int i = ; i < n; i++) a[i] = lower_bound(v.begin(), v.end(), a[i]) - v.begin() + ;
     return v.size();
 }
 int main()
 {
     scanf("%d", &n);
     for (int i = ; i < n; i++) scanf("%d %d", x + i, y + i);
     int X = compress(x), Y = compress(y);
     for (int i = ; i < n; i++) { lines[x[i]].push_back(y[i]); cnt[y[i]]++; }
     int ans = ;
     for (int i = ; i <= X; i++)
     {
         sort(lines[i].begin(), lines[i].end());
         for (int j = ; j < lines[i].size() - ; j++) ans += sum(lines[i][j + ] - ) - sum(lines[i][j]);
         for (int j = ; j < lines[i].size(); j++)
         {
             int tmp = lines[i][j];
             if (!vis[tmp]) { vis[tmp] = true; add(tmp, ); }
             now[tmp]++;
             if (now[tmp] == cnt[tmp]) add(tmp, -);
         }
     }
     printf("%d\n", ans + n);
     return ;
 }