Language: C++

#include <bits/stdc++.h>
using namespace std;
using ll = long long;

#define ALL(obj) (obj).begin(),(obj).end()
template<class T> using priority_queue_reverse = priority_queue<T,vector<T>,greater<T>>;

constexpr long long MOD = 1'000'000'000LL + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433L;

template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
vector<string> split(const string &str, const char delemiter) {vector<string> res;stringstream ss(str);string buffer; while( getline(ss, buffer, delemiter) ) res.push_back(buffer); return res;}
int msb(int x) {return x?31-__builtin_clz(x):-1;}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}

int main() {
    cin.tie(0);ios::sync_with_stdio(false);
    int H,W,K; cin >> H >> W >> K;
    auto grid = multivector(H*W,0);
    for(int i = 0; i < K; ++i) {
        ll y,x,v;
        cin >> y >> x >> v;
        y--,x--;
        grid[y*W+x]=v;
    }
    auto dp = multivector(H*W,2*4,0LL);
    //y
    dp[0*W+0][0][1]+=grid[0*W+0];
    //x
    dp[0*W+0][1][1]+=grid[0*W+0];
    for(int i = 0; i < H; ++i) {
        for(int j = 0; j < W; ++j) {

            //上から
            if(i) {
                for(int l=0; l<=3; ++l) {
                    //使わないとき
                    chmax(dp[i*W+j][0*4+l],dp[(i-1)*W+j][0*4+l]);
                    chmax(dp[i*W+j][1*4+0],dp[(i-1)*W+j][0*4+l]);
                    //使うとき
                    if(l+1<=3)chmax(dp[i*W+j][0*4+l+1],dp[(i-1)*W+j][0*4+l]+grid[i*W+j]);
                    if(l+1<=3)chmax(dp[i*W+j][1*4+1],dp[(i-1)*W+j][0*4+l]+grid[i*W+j]);
                }
            }
            //左から
            if(j) {
                for(int l=0; l<=3; ++l) {
                    //使わないとき
                    chmax(dp[i*W+j][0*4+0],dp[i*W+j-1][1*4+l]);
                    chmax(dp[i*W+j][1*4+l],dp[i*W+j-1][1*4+l]);
                    //使うとき
                    if(l+1<=3)chmax(dp[i*W+j][0*4+1],dp[i*W+j-1][1*4+l]+grid[i*W+j]);
                    if(l+1<=3)chmax(dp[i*W+j][1*4+l+1],dp[i*W+j-1][1*4+l]+grid[i*W+j]);
                }
            }

        }
    }
    ll ans = 0;
    for(int l=0; l<=3;++l) {
        chmax(ans,dp[(H-1)*W+W-1][0*4+l]);
        chmax(ans,dp[(H-1)*W+W-1][1*4+l]);
    }
    cout << ans << endl; 
    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:45:17: error: invalid types '__gnu_cxx::__alloc_traits<std::allocator<long long int>, long long int>::value_type {aka long long int}[int]' for array subscript
   45 |     dp[0*W+0][0][1]+=grid[0*W+0];
      |                 ^
a.cc:47:17: error: invalid types '__gnu_cxx::__alloc_traits<std::allocator<long long int>, long long int>::value_type {aka long long int}[int]' for array subscript
   47 |     dp[0*W+0][1][1]+=grid[0*W+0];
      |                 ^