Problem p03062 (C++)

Buggy submission (status: Time Limit Exceeded):
// I can

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

#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
#define pb push_back
#define ll long long
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep(i, n) for (i = 0; i < n; i++)
#define forn(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define for1(i, n) for (ll i = 1; i <= (ll)(n); ++i)
#define ford(i, n) for (ll i = (ll)(n)-1; i >= 0; --i)
#define fore(i, a, b) for (ll i = (ll)(a); i <= (ll)(b); ++i)
#define fora(it, x) for (auto it : x)
#define PI 3.14159265
#define sync                                                                   \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define endl "\n"

typedef pair<ll, ll> pii;
typedef vector<ll> vi;
typedef vector<pii> vpi;
typedef vector<vi> vvi;
typedef long long i64;
typedef vector<i64> vi64;
typedef vector<vi64> vvi64;
typedef pair<i64, i64> pi64;
typedef long double ld;

template <class T> bool uin(T &a, T b) { return a > b ? (a = b, true) : false; }
template <class T> bool uax(T &a, T b) { return a < b ? (a = b, true) : false; }

const ll maxn = 100000, mod = 1e9 + 7;

int n, a[maxn];
ll memo[maxn][2];

ll go(int pos, int par) {
  if (pos == n)
    return 0;

  if (memo[pos][par] != -1)
    return memo[pos][par];
  ll ans;
  if (par) {
    ll val = -a[pos];
    if (pos == n - 1)
      ans = val + go(pos + 1, 0);
    else
      ans = max(val + go(pos + 1, 0), a[pos] + go(pos + 1, 1));
  } else {
    if (pos == n - 1)
      ans = a[pos] + go(pos + 1, 0);
    else
      ans = max(a[pos] + go(pos + 1, 0), -a[pos] + go(pos + 1, 1));
  }
  return ans;
}

int main() {
  sync

      memset(memo, -1, sizeof(memo));
  cin >> n;
  forn(i, n) cin >> a[i];
  cout << go(0, 0);
  return 0;
}

Fixed submission:
// I can

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

#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
#define pb push_back
#define ll long long
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define rep(i, n) for (i = 0; i < n; i++)
#define forn(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define for1(i, n) for (ll i = 1; i <= (ll)(n); ++i)
#define ford(i, n) for (ll i = (ll)(n)-1; i >= 0; --i)
#define fore(i, a, b) for (ll i = (ll)(a); i <= (ll)(b); ++i)
#define fora(it, x) for (auto it : x)
#define PI 3.14159265
#define sync                                                                   \
  ios_base::sync_with_stdio(false);                                            \
  cin.tie(0);                                                                  \
  cout.tie(0);
#define endl "\n"

typedef pair<ll, ll> pii;
typedef vector<ll> vi;
typedef vector<pii> vpi;
typedef vector<vi> vvi;
typedef long long i64;
typedef vector<i64> vi64;
typedef vector<vi64> vvi64;
typedef pair<i64, i64> pi64;
typedef long double ld;

template <class T> bool uin(T &a, T b) { return a > b ? (a = b, true) : false; }
template <class T> bool uax(T &a, T b) { return a < b ? (a = b, true) : false; }

const ll maxn = 100000, mod = 1e9 + 7;

int n, a[maxn];
ll memo[maxn][2];

ll go(int pos, int par) {
  if (pos == n)
    return 0;

  if (memo[pos][par] != -1)
    return memo[pos][par];
  ll ans;
  if (par) {
    ll val = -a[pos];
    if (pos == n - 1)
      ans = val + go(pos + 1, 0);
    else
      ans = max(val + go(pos + 1, 0), a[pos] + go(pos + 1, 1));
  } else {
    if (pos == n - 1)
      ans = a[pos] + go(pos + 1, 0);
    else
      ans = max(a[pos] + go(pos + 1, 0), -a[pos] + go(pos + 1, 1));
  }
  return memo[pos][par] = ans;
}

int main() {
  sync

      memset(memo, -1, sizeof(memo));
  cin >> n;
  forn(i, n) cin >> a[i];
  cout << go(0, 0);
  return 0;
}