51Nod 1019 逆序数(线段树)

题目链接:逆序数

模板题。

 1 #include <bits/stdc++.h>
 2
 3 using namespace std;
 4
 5 #define rep(i, a, b) for (int i(a); i <= (b); ++i)
 6 #define lson i << 1, L, mid
 7 #define rson i << 1 | 1, mid + 1, R
 8
 9 const int N = 100010;
10
11 long long ans = 0;
12
13 struct node{
14     int x, y;
15     friend bool operator < (const node &a, const node &b){
16         return a.x < b.x;
17     }
18 } a[N];
19
20 int tree[N << 2];
21 int c[N];
22 int n;
23
24 inline void pushup(int i){
25     tree[i] = tree[i << 1] + tree[i << 1 | 1];
26 }
27
28 void build(int i, int L, int R){
29     tree[i] = 0;
30     if (L == R) return ;
31     int mid = (L + R) >> 1;
32     build(lson);
33     build(rson);
34 }
35
36 void update(int i, int L, int R, int pos, int val){
37     if (L == R && L == pos){
38         tree[i] += val;
39         return;
40     }
41
42     int mid = (L + R) >> 1;
43     if (pos <= mid) update(lson, pos, val);
44     else update(rson, pos, val);
45
46     pushup(i);
47 }
48
49 int query(int i, int L, int R, int l, int r){
50     if (L == l && R == r) return tree[i];
51     int mid = (L + R) >> 1;
52     if (r <= mid) return query(lson, l, r);
53     else if (l > mid) return query(rson, l, r);
54     else return query(lson, l, mid) + query(rson, mid + 1, r);
55 }
56
57 int main(){
58
59     scanf("%d", &n);
60
61     rep(i, 1, n){
62         scanf("%d", &a[i].x);
63         a[i].y = i;
64     }
65
66     sort(a + 1, a + n + 1);
67     c[a[1].y] = 1; rep(i, 2, n) c[a[i].y] = a[i].x == a[i - 1].x ? c[a[i - 1].y] : c[a[i - 1].y] + 1;
68
69     build(1, 1, n);  ans = 0;
70
71     rep(i, 1, n){
72         ans += (long long)query(1, 1, n, min(c[i] + 1, n), n);
73         update(1, 1, n, c[i], 1);
74     }
75
76     printf("%lld\n", ans);
77
78     return 0;
79 }