leetcode 188. Best Time to Buy and Sell Stock IV

class Solution {
public:
    int maxProfit(int k, vector<int> &prices){
        int n=prices.size(),v=0,p=0,ret=0;
        vector<int> profits;
        stack<pair<int,int>> vp;
        while(p<n){
            for(v=p;v<n-1&&prices[v]>=prices[v+1];++v);
            for(p=v+1;p<n&&prices[p]>=prices[p-1];++p);
            while(!vp.empty()&&prices[v]<prices[vp.top().first]){
                profits.push_back(prices[vp.top().second-1]-prices[vp.top().first]);
                vp.pop();
            }
            while(!vp.empty()&&prices[p-1]>=prices[vp.top().second-1]){
                profits.push_back(prices[vp.top().second-1]-prices[v]);
                v=vp.top().first;
                vp.pop();
            }
            vp.push(make_pair(v,p));
        }
        while(!vp.empty()){
            profits.push_back(prices[vp.top().second-1]-prices[vp.top().first]);
            vp.pop();
        }
        if(k>=profits.size()) return accumulate(profits.begin(),profits.end(),0);
        nth_element(profits.begin(),profits.end()-k,profits.end());
        return accumulate(profits.end()-k,profits.end(),0);
    } 
};