归并排序求逆序对个数-题解（C语言代码）

解题思路:归并排序 只不过加了个记录变量ans

注意事项:

参考代码:

#include<stdio.h>

long long ans = 0;

int a[500005], b[500005], n;

void merge(int low, int mid, int high) {

int i, j, k;

for (i = low, j = mid + 1, k = i; i <= mid && j <= high; k++) {

if (b[i] <= b[j])

a[k] = b[i++];

else {

a[k] = b[j++];

ans += mid - i + 1;

}

}

while (i <= mid) {

a[k++] = b[i++];

}

while (j <= high) {

a[k++] = b[j++];

}

for (i = low; i <= high; i++) {

b[i] = a[i];

}

}

void mergesort(int low, int high) {

if (low < high) {

int mid;

mid = (low + high) / 2;

mergesort(low, mid);

mergesort(mid + 1, high);

merge(low, mid, high);

}

}

int main() {

scanf("%d", &n);

int i;

for (i = 1; i <= n; i++) {

scanf("%d", &a[i]);

}

for (i = 1; i <= n; i++) {

b[i] = a[i];

}

mergesort(1, n);

printf("%lld", ans);

}