剑指OFFER----面试题51. 数组中的逆序对

链接：https://leetcode-cn.com/problems/shu-zu-zhong-de-ni-xu-dui-lcof/

代码

class Solution {
public:
    int merge_sort(int l, int r, vector<int>& nums) {
        if (l >= r) return 0;
        int mid = l + r >> 1;
        int res = merge_sort(l, mid, nums) + merge_sort(mid + 1, r, nums);
        int i = l, j = mid + 1, k = 0, help[nums.size() + 1];

        while (i <= mid && j <= r) {
            if (nums[i] <= nums[j]) {
                help[k++] = nums[i++];
            } else {
                res += mid - i + 1;
                help[k++] = nums[j++];
            }
        }

        while (i <= mid) help[k++] = nums[i++];
        while (j <= r) help[k++] = nums[j++];

        for (i = l, j = 0; i <= r; i++, j++) nums[i] = help[j];
        return res;
    }

    int reversePairs(vector<int>& nums) {
        int res = 0;
        return merge_sort(0, nums.size() - 1, nums);
    }
};