【逆序对】Ultra

POJ 2299 Ultra-QuickSort

只允许交换，比较相邻的元素， 求最少多少次交换可以使得序列有序

冒泡排序的次数——>数列中逆序对的个数减1——>最终为0 ——>答案为数列中逆序对的个数——> 归并排序求逆序对qwq

注意cnt开long long 不然会炸QAQ

板子！上！

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
const int sz = 500050;
int a[sz], b[sz], n;
long long cnt = 0;
void merge_sort(int l, int r) {
    if(r-l > 0) {//这里一开始写成while了QAQ 死循环无输出orz
        int mid = (l + r) >> 1;
        int i = l, p = l, q = mid+1;
        merge_sort(l, mid);
        merge_sort(mid+1, r);
        while(q<=r || p <= mid) {
            if(q > r || ((p <= mid)&&(a[p] <= a[q])))
                b[i++] = a[p++];
            else b[i++] = a[q++], cnt = cnt + mid - p + 1;
         }
         for(int i = l; i <= r; i++)
             a[i] = b[i];
    }
}
int main() {
    while(1) {
        scanf("%d", &n);
        if(n==0) break;
        memset(a, 0, sizeof(a));
        memset(b, 0, sizeof(b));
        for(int i = 1; i <= n; i++) 
            scanf("%d", &a[i]);
        cnt = 0;
        merge_sort(1, n);
        printf("%lld
", cnt);
    }
    
}