【剑指offer】数组中的逆序对

 1 class Solution {
 2 public:
 3     const int mod = 1000000007;
 4     long long int cnt = 0;
 5     void Merge(vector<int> &data,vector<int> &copy,int begin,int mid,int end){
 6         int start =  begin;
 7         int l = begin;   int r = mid + 1;
 8         while(l <= mid && r <= end){
 9             if(data[l] < data[r])   copy[begin++] = data[l++];
10             else{
11                 copy[begin++] = data[r++];
12                 cnt += mid - l + 1;
13             }
14         }
15         while(l <= mid) copy[begin++] = data[l++];
16         while(r <= end) copy[begin++] = data[r++];
17         //数组同一
18         while(start <= end){
19             data[start] = copy[start];
20             start++;
21         }
22     }
23     void MSort(vector<int> &data,vector<int> &copy,int begin,int end){
24         int mid;
25         if(begin < end){
26             mid = ( begin + end)/2;
27             MSort(data,copy,begin,mid);    //左
28             MSort(data,copy,mid+1,end);    //右
29             Merge(data,copy,begin,mid,end);    //合并
30         }
31     }
32     int InversePairs(vector<int> data) {
33         vector<int> copy = data;
34         int len = data.size();
35         MSort(data, copy, 0, len - 1);
36         return cnt % mod;
37     }
38 };