Add FZOI Codes

FZOI/求逆序对.cpp

@@ -0,0 +1,52 @@
+#include <bits/stdc++.h>
+#define int long long
+using namespace std;
+const int N = 5e5 + 10;
+int tree[N];
+
+void updata(int idx, int k)
+{
+	while(idx <= N)
+	{
+		tree[idx] += k;
+		idx += (idx) & (-idx);
+		
+	}
+}
+
+int query(int idx)
+{
+	int res = 0;
+	while(idx)
+	{
+		res += tree[idx];
+		idx -= (idx) & (-idx); 
+	} 
+	return res;
+}
+signed main()
+{
+	int n;
+	cin >> n ;
+	vector<int> a(n + 1);
+	for(int i = 1; i <= n; ++ i)
+	{
+		cin >> a[i];
+	}
+	vector<int> b = a;
+	sort(b.begin() + 1, b.end());
+	map<int, int> cnt;
+	int t = 1;
+	for(int i = 1; i <= n; ++ i)
+	{
+		if(!cnt.count(b[i])) cnt[b[i]] = t, t ++ ;
+	}
+	for(int i = 1; i <= n; ++ i) a[i] = cnt[a[i]];
+	int ans = 0;
+	for(int i = n; i >= 1; -- i)
+	{
+		updata(a[i], 1);
+		ans += query(a[i] - 1);
+	}
+	cout <<ans << endl ;
+}