Java 求整型数组的逆序数

如{7,5,6,4}的逆序数为5，可以使用归并排序来解决。

但是两个数组是从后向前进行比较，如果第一个数组的数字大于第二个数组当前的数字，就将第二个数组当前的数字位置数量就是对于数组1中当前数字的逆序数。

public class Main {

	public static void main(String[] args) throws Exception {

		int[] c = {7,5,6,4};
		System.out.println(findReverseCount(c));

	}

	public static int findReverseCount(int[] input) {
		if(input == null || input.length == 0){
			return 0;
		}
		int low = 0;
		int high = input.length-1;
		return findReverseCountHelper(input, low, high);
	}
	
	public static int findReverseCountHelper(int[] input, int low, int high){
		if(low == high){
			return 0;
		}
		int mid = (low+high)/2;
		int left = findReverseCountHelper(input, low, mid);
		int right = findReverseCountHelper(input, mid+1, high);
		int count = merge(input, mid, low, high);
		return left+right+count;
		
	}

	public static int merge(int[] input, int mid, int low, int high){
		int[] temp = new int[input.length+1];
		for(int i = low;i<=high;i++){
			temp[i] = input[i];
		}
		int leftEnd = mid;
		int rightEnd = high;
		int index = high;
		int count = 0;
		while(leftEnd>=low && rightEnd>=mid+1){
			if(temp[leftEnd]>temp[rightEnd]){
				count = count+rightEnd-mid;
				input[index] = temp[leftEnd];
				leftEnd--;
			}else{
				input[index] = temp[rightEnd];
				rightEnd--;
			}
			index--;
		}
		while(leftEnd>=low){
			input[index] = temp[leftEnd];
			leftEnd--;
			index--;
		}
		while(rightEnd>=mid+1){
			input[index] = temp[rightEnd];
			rightEnd--;
			index--;
		}
		return count++;
	}

}