class Solution {
public:
    int reversePairs(vector<int>& record) {
        if(record.size()<1)
            return 0;
        //归并 递归
        int left,right;
        left=0;
        right=record.size()-1;
        int num=mergeSort(left,right,record);
        return num;
    }
    int mergeSort(int left,int right, vector<int>& record){
        if(left>=right)
            return 0;
        int mid;
        mid=left+(right-left)/2;    
        int leftnum=mergeSort(left,mid,record);
        int rightnum=mergeSort(mid+1,right,record);  
        int mergenum=merge(left,right,mid,record);  
        return (leftnum+rightnum+mergenum);

    }
    int merge(int left,int right,int mid,vector<int>& record){
        int num=0;
        int i=left;
        int j=mid+1;
        vector<int> tmp;
        while(i<=mid && j<=right){
            if(record[i]>record[j]){
                num=mid-i+1+num;
                tmp.push_back(record[j++]);
            }
            else
                tmp.push_back(record[i++]);
        }

        while(i<=mid){
            tmp.push_back(record[i++]);
        }
        while(j<=right){
            tmp.push_back(record[j++]);
        }
        for(int i=0;i<tmp.size();i++){
            record[left+i]=tmp[i];
        }
        return num;
    }

};

class Solution {
public:
    int reversePairs(vector<int>& record) {
        //归并 迭代
        int num=0;
        
        for(int step=1;step<record.size();step*=2){
            int leftmin,leftmax,rightmin,rightmax;
            for(leftmin=0;leftmin+step<record.size();leftmin=rightmax){
                leftmax=rightmin=leftmin+step;
                rightmax=leftmax+step;
                if(rightmax>record.size())
                    rightmax=record.size();

                int start=leftmin;
                vector<int> tmp;
                while(leftmin<leftmax && rightmin<rightmax){
                    if(record[leftmin]<=record[rightmin]){
                        tmp.push_back(record[leftmin++]);
                    }
                    else{
                        tmp.push_back(record[rightmin++]);
                        num = num+leftmax-leftmin;
                    }
                }
                while(leftmin<leftmax){
                    tmp.push_back(record[leftmin++]);
                }
                while(rightmin<rightmax){
                    tmp.push_back(record[rightmin++]);
                }
                for(int i=0;i<tmp.size();i++){
                    record[start+i]=tmp[i];
                }
            }
        }
        return num;
    }
   

};