OfferWithLeetcodeJava/Solution51.java at master · leo-plus/OfferWithLeetcodeJava · GitHub

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class Solution {
    public int reversePairs(int[] nums) {
        if (nums == null || nums.length < 2) {
            return 0;
        }

        int[] tmp = new int[nums.length];

        return reversePairs(nums, 0, nums.length - 1, tmp);
    }

    private int reversePairs(int[] nums, int left, int right, int[] tmp) {
        if (left == right) {
            return 0;
        }

        int mid = left + (right - left) / 2;
        int leftPairs = reversePairs(nums, left, mid, tmp);
        int rightPairs = reversePairs(nums, mid + 1, right, tmp);

        int count = mergetAndCount(nums, left, mid, right, tmp);

        return leftPairs + rightPairs + count;
    }

    private int mergetAndCount(int[] nums, int left, int mid, int right, int[] tmp) {
        for (int i = left; i <= right; i++) {
            tmp[i] = nums[i];
        }

        int i = left;
        int j = mid + 1;

        int count = 0;

        for (int k = left; k <= right; k++) {
            if (i == mid + 1) {
                nums[k] = tmp[j];
                j++;
            } else if (j == right + 1) {
                nums[k] = tmp[i];
                i++;
            } else if (tmp[i] <= tmp[j]) {
                nums[k] = tmp[i];
                i++;
            } else {
                nums[k] = tmp[j];
                j++;
                count += mid - i + 1;
            }
        }

        return count;
    }
}
/**
 * https://leetcode-cn.com/problems/shu-zu-zhong-de-ni-xu-dui-lcof/solution/shu-zu-zhong-de-ni-xu-dui-by-leetcode-solution/
 */