Greetings 因为所有人都向右走，速度相同，因此如果两个人都还没有到达终点，那么这两个人不会相遇。只有一种情况会相遇，$i, j$ 两个人满足：$a_i > a_j, b_i < b_j$。可以把所有区间按照左端点从小到大排序，对于每一个 $b_i$，寻找之前所有 $> b_i$ 的区间的个数。把端点离散化之后，把点的值作为下标放到数组中，也就是求数组的后缀和，使用 $n + 1 - b_i$ 转换成求前缀和，可以使用树状数组，点更新、区间查询。
// Date: Thu Dec 28 23:25:57 2023 #include <climits> #include <cmath> #include <cstdio> #include <cstdlib> #include <cstring> #include <algorithm> #include <functional> #include <iomanip> #include <iostream> #include <map> #include <queue> #include <set> #include <sstream> #include <stack> #include <string> #include <utility> #include <vector> using namespace std; const int INF = 0x3f3f3f3f, MOD = 1e9 + 7; const double eps = 1e-8; const int dir[8][2] = { {0, 1}, {0, -1}, {1, 0}, {-1, 0}, {1, 1}, {1, -1}, {-1, 1}, {-1, -1}, }; typedef long long ll; typedef unsigned long long ull; typedef vector<int> VI; typedef pair<int, int> PII; const ull Pr = 131; #define For(i, a, b) for (int i = int(a); i < int(b); ++i) #define Rof(i, a, b) for (int i = int(b) - 1; i >= int(a); --i) #define For1(i, a, b) for (int i = int(a); i <= int(b); ++i) #define Rof1(i, a, b) for (int i = int(b); i >= int(a); --i) #define ForE(i, j) for (int i = h[j]; i != -1; i = ne[i]) #define f1 first #define f2 second #define pb push_back #define has(a, x) (a.find(x) != a.end()) #define nonempty(a) (!a.empty()) #define all(a) (a).begin(), (a).end() #define SZ(a) int((a).size()) template <typename t> istream &operator>>(istream &in, vector<t> &vec) { for (t &x : vec) in >> x; return in; } template <typename t> ostream &operator<<(ostream &out, vector<t> &vec) { int n = SZ(vec); For(i, 0, n) { out << vec[i]; if (i < n - 1) out << ' '; } return out; } #ifdef _DEBUG #define debug1(x) cout << #x " = " << x << endl; #define debug2(x, y) cout << #x " = " << x << " " #y " = " << y << endl; #define debug3(x, y, z) \ cout << #x " = " << x << " " #y " = " << y << " " #z " = " << z << endl; #else #define debug1 #define debug2 #define debug3 #endif const int N = 200010; int t, n, fen[N * 4]; inline int lowbit(int x) { return x & -x; } void update(int pos, int n) { while (pos <= n) { fen[pos]++; pos += lowbit(pos); } } int sum(int pos) { int result = 0; while (pos > 0) { result += fen[pos]; pos -= lowbit(pos); } return result; } int main(void) { #ifdef _DEBUG freopen("f.in", "r", stdin); #endif std::ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); cin >> t; while (t--) { cin >> n; memset(fen, 0, sizeof fen); unordered_map<int, int> mm; int idx = 0; vector<PII> a; VI bv; For1(i, 1, n) { int x, y; cin >> x >> y; a.pb({x, y}); bv.pb(y); } sort(all(bv)); For(i, 0, n) { mm[bv[i]] = ++idx; } sort(all(a)); set<int> s; ll res{}; for (auto &[x, y] : a) { int v = mm[y]; v = idx + 1 - v; int cnt = sum(v); res += cnt; update(v, idx); } cout << res << '\n'; } return 0; }