[leetCode]剑指 Offer 51. 数组中的逆序对 解法

可以使用归并排序的方法将数组分解成子数组，将子数组归并排序统计子数组的逆序数，最终统计整个数组的逆序数。

class Solution {
    public int reversePairs(int[] nums) {
        if(nums == null || nums.length == 0)
            return 0;
        int[] copy = Arrays.copyOf(nums, nums.length);
        int count = reversePairsCore( nums,  copy, 0, nums.length - 1);
        return count;
    }

    private int reversePairsCore(int[] nums, int[] copy, int start, int end) {
        if(start>=end) {
            copy[start] = nums[start];
            return 0;
        }
        int mid = start + (end - start)/2;
        int count = 0;
        int left = reversePairsCore(copy, nums, start, mid);
        int right = reversePairsCore(copy, nums, mid+1, end);
        int i = mid, j = end;
        for(int k = end; k >= start; k--) {
            if(j < mid + 1) copy[k] = nums[i--];
            else if(i < start) copy[k] = nums[j--];
            else if(nums[i] > nums[j]) {
                count += j - mid ;
                copy[k] = nums[i--];
            }else {
                copy[k] = nums[j--];
            }
        }
        return left + right + count;
    }
}