题解 | #数组中的逆序对#

class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param nums int整型vector 
     * @return int整型
     */
    const int kmod = 1000000007;
    int InversePairs(vector<int>& nums) {
        // write code here
        vector<int> tmp(nums.size());
        int res=0;
        mergeSort(nums,0,nums.size()-1,res,tmp);
        return res;
    }
    void merge(vector<int> &nums,int left,int mid,int right,int &res,vector<int> &tmp){
        //if(left==right) return ;
        int i=left,j=mid+1,k=0;
        while(i<=mid&&j<=right){
            if(nums[i]<=nums[j]){
                tmp[k++]=nums[i++];
            }else{
                tmp[k++]=nums[j++];
                res+=(mid-i+1);
                res%=kmod;
            }
        }
        while(i<=mid) tmp[k++]=nums[i++];
        while(j<=right) tmp[k++]=nums[j++];
        //tmp是复用的！！！！！！！
        for(int i=left,k=0;i<=right;i++,k++){
            nums[i]=tmp[k];
        }
    }
    void mergeSort(vector<int> &nums,int left,int right,int &res,vector<int> &tmp){
        if(left>=right) return;
        int mid=left+(right-left)/2;
        mergeSort(nums, left, mid, res,tmp);
        mergeSort(nums,mid+1,right,res,tmp);
        merge(nums,left,mid,right,res,tmp);
    }
};