【CDOJ931】Car race game（树状数组求逆序）

 1 #include <bits/stdc++.h>
 2 #define MAX 100010
 3 using namespace std;
 4 
 5 int s[MAX * 10];
 6 
 7 struct Node {
 8     int x, v;
 9 } node[MAX];
10 
11 bool cmp (Node a, Node b) {
12     if (a.x == b.x) {
13         return a.v > b.v;
14     }
15     return a.x < b.x;
16 }
17 
18 int lowbit (int x) {
19     return x & (-x);
20 }
21 
22 int sum (int p) {
23     int ans = 0;
24     while (p) {
25         ans += s[p];
26         p -= lowbit(p);
27     }
28     return ans;
29 }
30 
31 void add (int p, int del) {
32     while (p <= 1000000) {
33         s[p] += del;
34         p += lowbit(p);
35     }
36 }
37 
38 int main () {
39     int n, i;
40     while (scanf("%d", &n) != EOF) {
41         long long ans = 0;
42         for (i = 0; i < n; ++ i) {
43             scanf("%d %d", &node[i].x, &node[i].v);
44         }
45         sort (node, node + n, cmp);
46         memset(s, 0, sizeof(s));
47         for (i = 0; i < n; ++ i) {
48             ans = ans + i - sum(node[i].v);
49             add(node[i].v, 1);
50         }
51         printf ("%lld\n", ans);
52     }
53     return 0;
54 }