ZOJ-2386

来源：互联网发布：matlab 矩阵变cell 编辑：程序博客网时间：2024/05/16 13:07

问题可转化为求逆序对数，感觉用O(N^2)的排序算法会超时，没试过，我是用的归并排序，在排序的同时统计逆序对，注意merge的时候那个统计的逻辑，还有结果用long long 存，否则溢出，这题暂时排名第一，感觉良好，哈哈

#include<stdio.h>#include<string.h>static int array[500000];static long long res;static void merge(int *a, int begin, int mid, int end){int i = begin, j = mid + 1, index = 0;while (i <= mid && j <= end)if (a[i] <= a[j])array[index++] = a[i++];else{array[index++] = a[j++];res += mid - i + 1;//main point!!!}if (i > mid)while (j <= end)array[index++] = a[j++];if (j > end)while (i <= mid)array[index++] = a[i++];memcpy(a + begin, array, index * sizeof(int));}static void merge_sort(int *a, int begin, int end){if (begin == end)return;else if (begin + 1 == end){if (a[begin] > a[end]){int temp = a[begin];a[begin] = a[end];a[end] = temp;res++;}}else{int mid = (begin + end) / 2;merge_sort(a, begin, mid);merge_sort(a, mid + 1, end);merge(a, begin, mid, end);}}int main(){int n, a[500000];while (scanf("%d", &n), n){int i;for (i = 0; i < n; i++)scanf("%d", &a[i]);res = 0;merge_sort(a, 0, n - 1);printf("%lld\n", res);}return 0;}

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