【练习——逆序对】Ultra - Quicksort

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

板子！上！

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<iostream>
 4 using namespace std;
 5 const int sz = 500050;
 6 int a[sz], b[sz], n;
 7 long long cnt = 0;
 8 void merge_sort(int l, int r) {
 9     if(r-l > 0) {//这里一开始写成while了QAQ 死循环无输出orz
10         int mid = (l + r) >> 1;
11         int i = l, p = l, q = mid+1;
12         merge_sort(l, mid);
13         merge_sort(mid+1, r);
14         while(q<=r || p <= mid) {
15             if(q > r || ((p <= mid)&&(a[p] <= a[q])))
16                 b[i++] = a[p++];
17             else b[i++] = a[q++], cnt = cnt + mid - p + 1;
18          }
19          for(int i = l; i <= r; i++)
20              a[i] = b[i];
21     }
22 }
23 int main() {
24     while(1) {
25         scanf("%d", &n);
26         if(n==0) break;
27         memset(a, 0, sizeof(a));
28         memset(b, 0, sizeof(b));
29         for(int i = 1; i <= n; i++) 
30             scanf("%d", &a[i]);
31         cnt = 0;
32         merge_sort(1, n);
33         printf("%lld\n", cnt);
34     }
35     
36 }