模板——树状数组求逆序对

题目链接：https://www.luogu.org/problemnew/show/P1908

 1 #include <map>
 2 #include <set>
 3 #include <cmath>
 4 #include <queue>
 5 #include <string>
 6 #include <cstdio>
 7 #include <cstring>
 8 #include <iostream>
 9 #include <algorithm>
10 #define forn(i, n) for (int i = 0; i < (n); i++)
11 #define forab(i, a, b) for (int i = (a); i <= (b); i++)
12 #define forba(i, b, a) for (int i = (b); i >= (a); i--)
13 #define mset(a, n) memset(a, n, sizeof(a))
14 #define fast ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
15 #define P pair<int,int>
16 #define fi first
17 #define se second
18 using namespace std;
19 #define N 500005
20 #define maxn 1005
21 #define inf 0x3f3f3f3f
22 #define ll long long
23 ll a[N], c[N * 4];
24 ll b[N];
25 int n;
26 int sum;
27 inline int lowbit(int x) { return x & (-x); }
28 void add(int x,int y)
29 {
30     for (; x <= N;x+=lowbit(x))
31         c[x] += y;
32 }
33 int ask(int x)
34 {
35     int ans = 0;
36     for (; x;x-=lowbit(x))
37         ans += c[x];
38     return ans;
39 }
40 bool cmp(int x,int y)
41 {
42     return b[x] > b[y];
43 }
44 int main()
45 {
46     scanf("%d", &n);
47     forab(i, 1, n)
48     {
49         scanf("%d", b + i);
50         a[i] = i;
51     }
52     sort(a + 1, a + 1 + n, cmp);
53    // forab(i, 1, n) cout << a[i] << " ";
54    // cout << endl;
55     forab(i, 1, n)
56     {
57         
58        // cout << "sum=" << sum << endl;
59         add(a[i], 1);
60         sum += ask(a[i] - 1);
61     }
62     cout << sum << endl;
63     system("pause");
64 }
65 /*
66 6
67 5 4 2 6 3 1
68 
69 */