归并排序 + 逆序数

 

 

#include <cstdio>
#include <iostream>

using namespace std;

int ans = 0;

void merge(int* l, int* mid, int *r)
{
    int b[r-l];
    int *i = l, *j = mid, cur = 0;
    while(i<mid || j<r) {
        if(i<mid && (j>=r || *i <= *j))    b[cur++] = *i++;
        else {
            b[cur++] = *j++;
            ans += mid - i; 
        } 
    }
    for(int *p=l, i=0; p < r; p++, i++)    *p = b[i];
}

void merge_sort(int *l, int *r) // [ , )
{
    if(r-l==1)    return;
    int* mid = l+(r-l)/2;
    merge_sort(l, mid);
    merge_sort(mid, r);
    merge(l, mid, r);
}

int main ()
{
    int a[] = {6,2,1,8,7,5,3,4};
//    int a[] = {2,4,3,1};
    int n = sizeof(a) / sizeof(int);
    ans = 0;
    merge_sort(a, a+n);
    for(int i=0; i<n; i++)    printf("%d ", a[i]);
    printf("\n"); 
    printf("逆序数为：%d\n", ans);
    return 0;
}