poj 2299 Ultra-QuickSort

http://162.105.81.212/JudgeOnline/problem?id=2299

 

归并排序, 在"归并"时计算逆序数的个数.

ret += mid-first1+1;

 

#include<iostream> #include<algorithm> using namespace std; #define MAX 500000 __int64 ret; int a[MAX], tmp[MAX]; void Merge(int *a, int first, int mid, int last) { int first1, last1, first2, last2, k, i; first1 = first, last1 = mid; first2 = mid + 1, last2 = last; k = 0; while(first1 <= last1 && first2 <= last2) { if(a[first1] < a[first2]) tmp[k++] = a[first1++]; else { tmp[k++] = a[first2++]; ret += mid-first1+1; } } while(first1 <= last1) tmp[k++] = a[first1++]; while(first2 <= last2) tmp[k++] = a[first2++]; for(i=0; i<(last-first+1); i++) a[first+i] = tmp[i]; } void MergeSort(int *a, int first, int last) { int mid = 0; if(first < last) { mid = (first+last)>>1; MergeSort(a, first, mid); MergeSort(a, mid+1, last); Merge(a, first, mid ,last); } } int main() { int n, i; while(scanf("%d", &n), n) { for(i=0; i<n; i++) scanf("%d", a+i); ret = 0; MergeSort(a, 0, n-1); printf("%I64d/n", ret); } return 0; }