剑指 Offer 51. 数组中的逆序对

public class Num51数组中的逆序对重要 {
    int count = 0;

    public int reversePairs(int[] nums) {
        mergeSort(nums, 0, nums.length - 1, new int[nums.length]);
        return count;
    }

    private void mergeSort(int[] nums, int left, int right, int[] temp) {
        if (left >= right) return;
        int mid = left + (right - left) / 2;
        mergeSort(nums, left, mid, temp);
        mergeSort(nums, mid + 1, right, temp);
        merge(nums, left, mid, right, temp);
    }

    private void merge(int[] nums, int left, int mid, int right, int[] temp) {
        int i = left, j = mid + 1, t = left;
        while (i <= mid && j <= right) {
            if (nums[i] <= nums[j]) {
                temp[t++] = nums[i++];
            } else {
                temp[t++] = nums[j++];
                count += mid - i + 1;
            }
        }
        while (i <= mid) temp[t++] = nums[i++];
        while (j <= right) temp[t++] = nums[j++];
        for (int k = left; k <= right; k++) {
            nums[k] = temp[k];
        }
    }
}