面试：分而治之求逆序数

在并归排序的基础上实现逆序数计算，时间复杂度为O(N*log2N)

<span style="font-size:18px;">#include <iostream>
using namespace std;

int ret = 0;

template <typename T>
void mergeAdjust(T *target, T *src, int start, int mid, int end) {
	int temp = 0;
	int i = start;
	int j = mid + 1;
	int cnt = start;
	while(i <= mid && j <= end) {
		if(src[i] > src[j]) {
			target[cnt ++] = src[j ++];
			temp += mid - i + 1;
		} else {
			target[cnt ++] = src[i ++];
		}
	}
	while(i <= mid) {
		target[cnt ++] = src[i ++];
	}
	while(j <= end) {
		target[cnt ++] = src[j ++];
	}
	ret += temp;
}

template <typename T>
void mergeSort(T *target, T *src, int start, int end) {
	if(start == end) {
		target[start] = src[start];
		return;
	}
	T buf[100];
	int mid = start + (end - start) / 2;
	mergeSort(buf, src, start, mid);
	mergeSort(buf, src, mid + 1, end);
	mergeAdjust(target, buf, start, mid, end);
}

int main(void) {
	//int a[] = {9, 8, 7, 6, 5, 4, 3, 2, 1};
	int a[] = {3, 2, 1, 4};
	int b[100];
	mergeSort(b, a, 0, 3);
	cout << ret << endl;
	return 0;
}</span>