P1908-逆序对 

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define _for(i,a,b) for(int i = (a);i < b;i ++)
 4 const int maxn = 500009;
 5 typedef long long ll;
 6 int n;
 7 ll a[maxn],c[maxn];
 8 ll ans = 0;
 9 inline ll read()
10 {
11     ll ans = 0;
12     char ch = getchar(), last = ' ';
13     while(!isdigit(ch)) last = ch, ch = getchar();
14     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
15     if(last == '-') ans = -ans;
16     return ans;
17 }
18 inline void write(ll x)
19 {
20     if(x < 0) x = -x, putchar('-');
21     if(x >= 10) write(x / 10);
22     putchar(x % 10 + '0');
23 }
24 void msort(int le,int rr)
25 {
26     if(le==rr)
27         return ;
28     int mid = (le+rr)/2;
29     msort(le,mid),msort(mid+1,rr);
30     int i = le,j = mid+1;
31     int k = le;
32     while(i <= mid && j <= rr)
33     {
34         if(a[i]<=a[j])
35             c[k++] = a[i++];
36         else
37         {
38             c[k++] = a[j++];
39             ans += mid-i+1;
40         }
41     }
42     memcpy(c+k,a+i,(mid-i+1)*(sizeof(ll)));
43     k += mid-i+1;
44     memcpy(c+k,a+j,(rr-j+1)*(sizeof(ll)));
45     memcpy(a+le,c+le,(rr-le+1)*(sizeof(ll)));
46 }
47 int main()
48 {
49     n = read();
50     _for(i,0,n)
51     a[i] = read();
52 
53     msort(0,n-1);
54     write(ans);
55     return 0;
56 }

 

posted @ 2019-07-28 12:39  Asurudo  阅读(155)  评论(0)    收藏  举报