[模板]排序

1.快速排序

#include <iostream>
using namespace std;

const int MAXN = 2e5 + 10;

int arr[MAXN];

int len;

void qsort(int arr[], int fst, int lst)
{
    int i = fst, j = lst;
    
    int mid = arr[(i + j) / 2];
    
    while(i <= j)
    {
        while(arr[i] < mid) //更改符号换递减序
            i ++;
       
        while(arr[j] > mid) //同上
            j --;
        
        if(i <= j)
        {
            swap(arr[i], arr[j]);
            i ++; j --;
        }
    }
    if(i < lst)
        qsort(arr,i, lst);
    
    if(fst < j)
        qsort(arr,fst, j);
}
int main()
{
	ios::sync_with_stdio(false);
    
    cin>>len;
    
    for(int i = 0; i < len; i++)
        cin>>arr[i];

    qsort(arr, 0, len - 1);

    for(int i = 0; i < len; i++)
        cout<<arr[i]<<' ';
    
    cout<<endl;
    
    return 0;
}

2.归并排序

求逆序数

#include <iostream>
using namespace std;

typedef long long ll;

const int MAXN = 1e5 + 10;

ll arr[MAXN];

ll brr[MAXN];

ll ans = 0;

ll N, x, y; 

void merge_sort(ll arr[], ll x, ll y, ll brr[])
{
    if(y - x > 1)
    {
        int m = x + (y - x) / 2;
        
        int p = x, q= m, i = x;
        
        merge_sort(arr, x, m, brr);
       
        merge_sort(arr, m, y, brr);
        
        while(p < m || q < y)
        {
            if(q >= y || (p < m && arr[p] <= arr[q]))
            {
                brr[i++] = arr[p++]; 
            }
            
            else
            {
                brr[i++] = arr[q++];
                ans += m - p; //求逆序数
            }
        }
        
        for(i = x; i < y; i++)
            arr[i] = brr[i];
    }
}

int main()
{
    ios::sync_with_stdio(false);

    while(cin>>N)
    {
        ans = 0;

        for(int i = 0; i < N; i++)
            cin>>arr[i];

        merge_sort(arr, 0, N, brr);

        cout<<ans<<endl;  
    }

    return 0;
}