java，利用归并排序模板求解逆序对

//归并排序求逆序对
class Solution {
    static int res;
    public int inversePairs(int[] nums) {
        mergeSort(nums, 0, nums.length - 1);
        return res;
    }

    public void mergeSort(int[] nums, int l, int r) {
        if(l >= r) return;
        int mid = l + r >> 1;
        mergeSort(nums, l, mid);
        mergeSort(nums, mid + 1, r);
        int k = 0;
        int[] temp = new int[r - l + 1];
        int i = l, j = mid + 1;
        while(i <= mid && j <= r) {
            if(nums[i] <= nums[j]) {
                temp[k++] = nums[i++];
            } else{
                res += mid - i + 1;
                temp[k++] = nums[j++];
            }
        }
        while(i <= mid) temp[k++] = nums[i++];
        while(j <= r) temp[k++] = nums[j++];
        for(i = l, j = 0; i <= r; i++, j++) nums[i] = temp[j];
    }
}