HackerRank

First I thought it should be solved using DP, and I gave a standard O(n^2) solution:

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
using namespace std;

#define REP(i, s, n) for(int i = s; i < n; i ++)
typedef long long LL;

LL calc(vector<LL> &in)
{
    size_t len = in.size();

    LL ret = 0;

    /*
    //    dp[i][onhand]
    vector<vector<LL>> dp(len, vector<LL>(len + 1, std::numeric_limits<LL>::min()));
    dp[0][0] = 0; // no action
    dp[0][1] = -in[0]; // buy

    REP(i, 1, len)
    REP(j, 0, i + 1)
    {
        //    Choice 1: buy
        dp[i][j + 1] = std::max(dp[i][j + 1], dp[i - 1][j] - in[i]); 
        //    Choice 2: no action
        dp[i][j] = std::max(dp[i][j], dp[i - 1][j]);
        //    Choice 3: sell all
        if(j > 0)
            dp[i][0] = std::max(dp[i][0], in[i] * j + dp[i-1][j]);        
    }
    ret = *std::max_element(dp[len-1].begin(), dp[len-1].end());
    */
    vector<LL> pre(len + 1, std::numeric_limits<LL>::min());
    pre[0] = 0; pre[1] = -in[0];
    vector<LL> now(len + 1, std::numeric_limits<LL>::min());

    vector<LL> *ppre = &pre, *pnow = &now;
    REP(i, 1, len)
    {
        REP(j, 0, i + 1)
        {
            //    Choice 1: buy
            (*pnow)[j + 1] = std::max((*pnow)[j + 1], (*ppre)[j] - in[i]); 
            //    Choice 2: no action
            (*pnow)[j] = std::max((*pnow)[j], (*ppre)[j]);
            //    Choice 3: sell all
            if(j > 0)
                (*pnow)[0] = std::max((*pnow)[0], in[i] * j + (*ppre)[j]);    
        }
        // swap
        std::swap(ppre, pnow);
        (*pnow).assign(len + 1, std::numeric_limits<LL>::min());
    }
    ret = *std::max_element((*ppre).begin(), (*ppre).end());
    return ret;
}

int main()
{
    int t; cin >> t;
    while(t--)
    {
        int n; cin >> n;
        vector<LL> in(n);
        REP(i, 0, n) cin >> in[i];
        cout << calc(in) << endl;
    }
    return 0;
}

But all TLE.. so there are must be a O(n) solution, and there is.. what is better than a standard DP in cerntain cases? Greedy.

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
using namespace std;

#define REP(i, s, n) for(int i = s; i < n; i ++)
typedef long long LL;

LL calc(vector<LL> &in)
{
    size_t len = in.size();

    LL ret = 0;
    std::reverse(in.begin(), in.end());
    LL peak = -1;
    REP(i, 0, len)
    {
        if(in[i] > peak)
        {
            peak = in[i];
        }
        else
        {
            ret += peak - in[i];
        }
    }
    return ret;
}

int main()
{
    int t; cin >> t;
    while(t--)
    {
        int n; cin >> n;
        vector<LL> in(n);
        REP(i, 0, n) cin >> in[i];
        cout << calc(in) << endl;
    }
    return 0;
}