1 files changed, 8 insertions, 26 deletions
diff --git a/math/inversions.cpp b/math/inversions.cpp
index 0720407..02256ca 100644
--- a/math/inversions.cpp
+++ b/math/inversions.cpp
@@ -1,27 +1,9 @@
-// Laufzeit: O(n*log(n))
-ll merge(vector<ll> &v, vector<ll> &left, vector<ll> &right) {
-  int a = 0, b = 0, i = 0;
-  ll inv = 0;
-  while (a < (int)left.size() && b < (int)right.size()) {
-    if (left[a] < right[b]) v[i++] = left[a++];
-    else {
-      inv += left.size() - a;
-      v[i++] = right[b++];
-    }
-  }
-  while (a < (int)left.size()) v[i++] = left[a++];
-  while (b < (int)right.size()) v[i++] = right[b++];
-  return inv;
-}
-
-ll mergeSort(vector<ll> &v) { // Sortiert v und gibt Inversionszahl zurück.
-  int n = v.size();
-  vector<ll> left(n / 2), right((n + 1) / 2);
-  for (int i = 0; i < n / 2; i++) left[i] = v[i];
-  for (int i = n / 2; i < n; i++) right[i - n / 2] = v[i];
-
-  ll result = 0;
-  if (left.size() > 1) result += mergeSort(left);
-  if (right.size() > 1) result += mergeSort(right);
-  return result + merge(v, left, right);
+ll inversions(const vector<ll>& v) {
+	Tree<pair<ll, ll>> t; //ordered statistics tree
+	ll res = 0;
+	for (ll i = 0; i < (ll)v.size(); i++) {
+		res += i - t.order_of_key({v[i], i});
+		t.insert({v[i], i});
+	}
+	return res;
 }