算法导论（Exercise 2-4）

#include<iostream>
#include<vector>
#include<string>
#include<set>
#include<map>
#include<unordered_set>
#include<unordered_map>
#include<algorithm>
#include<xfunctional>
using namespace std;

int Merge_sort(vector<int>& arr,int start,int mid,int end){
	vector<int>  left(arr.begin()+start,arr.begin()+mid+1);
	vector<int>  right(arr.begin()+mid+1,arr.begin()+end+1);
	int index = start;
	int count = 0;
	int i = 0, j = 0;
	while (i < left.size() && j < right.size()){
		if (left[i] >= right[j]){
			count += (left.size()-i);
			arr[index] = right[j];
			j++;
			index++;
		}else{
			arr[index] = left[i];
			i++;
			index++;
		}
	}
	while (i < left.size()){
		arr[index] = left[i];
		i++;
		index++;
	}
	while (j < right.size()){
		arr[index] = right[j];
		j++;
		index++;
	}
	return count;
}

int Merge_count(vector<int>& arr,int start,int end){
	if (start >= end) return 0;
	int mid = (start+end) / 2;
	int left = Merge_count(arr,start,mid);
	int right = Merge_count(arr,mid+1,end);
	int cur = Merge_sort(arr,start,mid,end);
	return left + cur + right;
}

int main(){
	int n; 
	cout << "Input the size(>=1) of the array:";
	cin >> n;
	vector<int> arr(n,0);
	cout << "Input the array element:";
	for (int i = 0; i < n; i++) cin >> arr[i];
	cout <<"The number of reverse order:";
	int sum = Merge_count(arr,0,arr.size()-1);
	cout <<sum<< endl;
	system("pause");
	return 0;
}