wikioi 1688--求逆序对

题目描述：给定数组，求逆序对的个数

思路：归并排序，归并的时候改变计数，当前面的元素比后面元素大则计数cnt+=(m-i)+1

没有AC的版本

 1 #include <iostream>
 2 #include <queue>
 3 #include <climits>
 4 #include <algorithm>
 5 #include <memory.h>
 6 #include <stdio.h>
 7 #include <ostream>
 8 #include <vector>
 9 #include <list>
10 #include <cmath>
11 #include <string>
12 #include <stdexcept>
13 #include <stack>
14 #include <map>
15 using namespace std;
16 
17 vector<int> a;
18 vector<int> tmpa;
19 int ans;
20 void mergeArray(int l,int mid,int r)
21 {
22     int i = l;
23     int j = mid+1;
24     int k = l;
25     while(i <= mid && j <=r)
26     {
27         if(a[i] > a[j])
28         {
29             tmpa[k++] = a[j++];
30             ans += mid-i+1;
31         }
32         else
33         {
34             tmpa[k++] = a[i++];
35         }
36     }
37     while(i <= mid)
38         tmpa[k++] = a[i++];
39     while(j <= r)
40         tmpa[k++] = a[j++];
41     for(i = l;i<=r;++i)
42         a[i] = tmpa[i];
43 }
44 void merge_sort(int l,int r)
45 {
46     if(l<r)
47     {
48         int mid = (l+r)/2;
49         merge_sort(l,mid);
50         merge_sort(mid+1,r);
51         mergeArray(l,mid,r);
52     }
53 }
54 
55 int main()
56 {
57     ans = 0;
58     int n;
59     cin>>n;
60     for(int i = 0 ; i < n ; ++i)
61     {
62         int tmp;
63         cin>>tmp;
64         a.push_back(tmp);
65         tmpa.push_back(tmp);
66     }
67     merge_sort(0,n-1);
68     cout<<ans<<endl;
69     return 0;
70 }