归并排序

复杂度：O(nlogn)

一种稳定的排序算法，分治思想

#include<bits/stdc++.h>

using namespace std;

int a[500],t[500];

void merge(int l,int r){
	if(l == r) return;
	int mid = (l+r)/2;
	merge(l,mid);
	merge(mid+1,r);
	int i = l,j = mid+1,k = l;
	while(i<=mid&&j<=r){
		if(a[i] <= a[j]){
			t[k++] = a[i++];
		}else{
			t[k++] = a[j++];
		}
	}
	while(i<=mid) t[k++] = a[i++];
	while(j<=r) t[k++] = a[j++];
	for(int i=l;i<=r; i++){
		a[i] = t[i];
	}
}

int main(){
	int n;
	cin >> n;
	for(int i=1; i<=n; i++){
		cin >> a[i];
	}
	merge(1,n);
	for(int i=1; i<=n; i++){
		cout << a[i] << " ";
	}
	return 0;
}

　　

求逆序对：洛谷P1908