POJ2299 Ultra-QuickSort
原题传送:http://poj.org/problem?id=2299
归并排序求逆序数,nlog(n),500多ms。答案最大可以达到n * (n + 1) / 2,要开long long。
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 #include <stdio.h> 2 #include <stdlib.h> 3 #define N 500005 4 #define LL __int64 5 LL a[N], b[N]; 6 int n; 7 8 LL cal(LL s, LL t) 9 { 10 LL i, j, k, mid; 11 LL res; 12 if(t - s == 1) 13 return 0; 14 mid = (s + t) >> 1; 15 res = cal(s, mid) + cal(mid, t); 16 i = s, j = mid, k = s; 17 while(i < mid && j < t) 18 { 19 if(a[i] <= a[j]) 20 b[k ++] = a[i ++], res += j - mid; 21 else 22 b[k ++] = a[j ++]; 23 } 24 while(i < mid) b[k ++] = a[i ++], res += j - mid; 25 while(j < t) b[k ++] = a[j ++]; 26 for(i = 0; i < t - s; i ++) 27 a[s + i]= b[s + i]; 28 return res; 29 } 30 31 int main() 32 { 33 while(scanf("%d", &n), n) 34 { 35 for(LL i = 0; i < n; i ++) 36 scanf("%I64d", &a[i]); 37 printf("%I64d\n", cal(0, n)); 38 } 39 return 0; 40 }
POJ1007用的是一样的算法。