327. Count of Range Sum

class Solution {private:    int countHelper(vector<long>&sum,int start,int end,int lower,int upper)    {        if(end-start<=1)//sum个数小于等于1无法求范围            return 0;        int mid=(start+end)/2;        int count=countHelper(sum,start,mid,lower,upper)+countHelper(sum,mid,end,lower,upper);        vector<long> cache(end-start,0);        int k=mid;//第一个sum[k]-sum[i]>=lower        int j=mid;//第一个sum[j]-sum[i]>upper        int t=mid;//sum[t]<sum[i];        int len=0;        for(int r=0,i=start;i<mid;i++)        {            while(k<end&&sum[k]-sum[i]<lower)k++;            while(j<end&&sum[j]-sum[i]<=upper)j++;            count+=(j-k);//更新count            while(t<end&&sum[t]<sum[i])cache[r++]=sum[t++];            cache[r++]=sum[i];            len=r;        }        for(int i=0;i<len;i++)        {            sum[start+i]=cache[i];        }        return count;    }public:    int countRangeSum(vector<int>& nums, int lower, int upper) {        if(nums.size()==0)            return 0;        vector<long> sum(nums.size()+1,0);        for(int i=1;i<=nums.size();i++)        {            sum[i]=sum[i-1]+nums[i-1];        }        return countHelper(sum,0,nums.size()+1,lower,upper);    }};