Problem p03165 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, m) for (long long i = 0; i < m; i++)
#define per(i, m) for (long long i = m - 1; i >= 0; i--)
#define FOR(i, n, m) for (long long i = n; i < m; i++)
#define ROF(i, n, m) for (long long i = m - 1; i >= n; i--)
#define SORT(v, n)                                                             \
  do {                                                                         \
    sort(v, v + n);                                                            \
    reverse(v, v + n);                                                         \
  } while (0)
#define all(x) (x).begin(), (x).end()
#define EPS (1e-7)
#define INF (1e18)
#define PI (acos(-1))
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
typedef pair<ll, ll> LP;

ll POW(ll x, ll n) {
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return POW(x * x, n / 2) % MOD;
  return x % MOD * POW(x, n - 1) % MOD;
}

ll POW2(ll x, ll n) {
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return POW2(x * x, n / 2);
  return x * POW2(x, n - 1);
}

ll POW3(ll x, ll n, ll m) {
  x %= m;
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return POW3(x * x, n / 2, m) % m;
  return x * POW3(x, n - 1, m) % m;
}

ll gcd(ll u, ll v) {
  ll r;
  while (0 != v) {
    r = u % v;
    u = v;
    v = r;
  }
  return u;
}

ll lcm(ll u, ll v) { return u * v / gcd(u, v); }

ll KAI(ll m) {
  if (m < 0)
    return 0;
  if (m == 0)
    return 1;
  return m * KAI(m - 1) % MOD;
}

ll KAI2(ll m) {
  if (m < 0)
    return 0;
  if (m == 0)
    return 1;
  return m * KAI2(m - 1);
}

ll extGCD(ll a, ll b, ll &x, ll &y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  ll d = extGCD(b, a % b, y, x);
  y -= a / b * x;
  return d;
}

inline ll mod(ll a, ll m) { return (a % m + m) % m; }

ll modinv(ll a) {
  ll x, y;
  extGCD(a, MOD, x, y);
  return mod(x, MOD);
}

ll COM(ll m, ll n) {
  if (m < n)
    return 0;
  if (n < 0)
    return 0;
  if (n == 0)
    return 1;
  if (m == n)
    return 1;
  return KAI(m) % MOD * modinv(KAI(n) % MOD * KAI(m - n) % MOD) % MOD;
}

ll COM2(ll m, ll n) {
  if (m < n)
    return 0;
  if (n < 0)
    return 0;
  if (n == 0)
    return 1;
  if (m == n)
    return 1;
  return KAI2(m) / KAI2(n) / KAI2(m - n);
}

ll DEC(ll x, ll m, ll n) // xのm進数でのx^nの位の値
{
  return x % POW2(m, n + 1) / POW2(m, n);
}

ll keta(ll x, ll n) // xのn進数での桁数
{
  if (x == 0)
    return 0;
  return keta(x / n, n) + 1;
}

ll DIV(ll x, ll n) // x!のnで割り切れる回数
{
  if (x == 0)
    return 0;
  return x / n + DIV(x / n, n);
}

ll ORD(ll x, ll n) // xのnで割り切れる回数
{
  if (x == 0)
    return INF;
  if (x % n != 0)
    return 0;
  return 1 + ORD(x / n, n);
}

ll SGS(ll x, ll y, ll m) // 1+x+…+x^(y-1)をmで割った余り
{
  if (y == 0)
    return 0;
  if (y % 2 == 0) {
    return (1 + POW3(x, y / 2, m)) * SGS(x, y / 2, m) % m;
  }
  return (1 + x * SGS(x, y - 1, m)) % m;
}

ll SSGS(ll x, ll y, ll m) // Σ[k=1→y](1+x+…+x^(k-1))をmで割った余り
{
  if (y == 0)
    return 0;
  if (y == 1)
    return 1;
  if (y % 2 == 0) {
    return (SSGS(x, y / 2, m) * (POW3(x, y / 2, m) + 1) % m +
            SGS(x, y / 2, m) * y / 2 % m) %
           m;
  }
  return (SSGS(x, y - 1, m) * x % m + y) % m;
}

struct UnionFind {
  vector<int> par;
  vector<int> sizes;
  UnionFind(int n) : par(n), sizes(n, 1) { rep(i, n) par[i] = i; }
  int find(int x) {
    if (x == par[x])
      return x;
    return par[x] = find(par[x]);
  }
  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (sizes[x] < sizes[y])
      swap(x, y);
    par[y] = x;
    sizes[x] += sizes[y];
  }
  bool same(int x, int y) { return find(x) == find(y); }
  int size(int x) { return sizes[find(x)]; }
};

int main() {
  string s, t;
  ll dp[320][320], m[320][320], a, b;
  stack<char> p;
  cin >> s >> t;
  rep(i, s.length()) {
    rep(j, t.length()) {
      if (s[i] == t[j]) {
        dp[i + 1][j + 1] = dp[i][j] + 1;
        m[i + 1][j + 1] = 0;
      }
      dp[i + 1][j + 1] = max({dp[i + 1][j], dp[i][j + 1], dp[i + 1][j + 1]});
      if (dp[i + 1][j + 1] == dp[i + 1][j]) {
        m[i + 1][j + 1] = 1;
      }
      if (dp[i + 1][j + 1] == dp[i][j + 1]) {
        m[i + 1][j + 1] = 2;
      }
    }
  }
  a = s.length();
  b = t.length();
  while (a >= 1 && b >= 1) {
    if (m[a][b] == 0) {
      p.push(s[a - 1]);
      a--;
      b--;
    } else if (m[a][b] == 1)
      b--;
    else
      a--;
  }
  while (p.size()) {
    cout << p.top();
    p.pop();
  }
}

Fixed submission:
#include <bits/stdc++.h>
#define rep(i, m) for (long long i = 0; i < m; i++)
#define per(i, m) for (long long i = m - 1; i >= 0; i--)
#define FOR(i, n, m) for (long long i = n; i < m; i++)
#define ROF(i, n, m) for (long long i = m - 1; i >= n; i--)
#define SORT(v, n)                                                             \
  do {                                                                         \
    sort(v, v + n);                                                            \
    reverse(v, v + n);                                                         \
  } while (0)
#define all(x) (x).begin(), (x).end()
#define EPS (1e-7)
#define INF (1e18)
#define PI (acos(-1))
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \
       << " " << __FILE__ << endl;
using namespace std;
typedef long long ll;
const ll MOD = 1000000007;
typedef pair<ll, ll> LP;

ll POW(ll x, ll n) {
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return POW(x * x, n / 2) % MOD;
  return x % MOD * POW(x, n - 1) % MOD;
}

ll POW2(ll x, ll n) {
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return POW2(x * x, n / 2);
  return x * POW2(x, n - 1);
}

ll POW3(ll x, ll n, ll m) {
  x %= m;
  if (n == 0)
    return 1;
  if (n % 2 == 0)
    return POW3(x * x, n / 2, m) % m;
  return x * POW3(x, n - 1, m) % m;
}

ll gcd(ll u, ll v) {
  ll r;
  while (0 != v) {
    r = u % v;
    u = v;
    v = r;
  }
  return u;
}

ll lcm(ll u, ll v) { return u * v / gcd(u, v); }

ll KAI(ll m) {
  if (m < 0)
    return 0;
  if (m == 0)
    return 1;
  return m * KAI(m - 1) % MOD;
}

ll KAI2(ll m) {
  if (m < 0)
    return 0;
  if (m == 0)
    return 1;
  return m * KAI2(m - 1);
}

ll extGCD(ll a, ll b, ll &x, ll &y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  ll d = extGCD(b, a % b, y, x);
  y -= a / b * x;
  return d;
}

inline ll mod(ll a, ll m) { return (a % m + m) % m; }

ll modinv(ll a) {
  ll x, y;
  extGCD(a, MOD, x, y);
  return mod(x, MOD);
}

ll COM(ll m, ll n) {
  if (m < n)
    return 0;
  if (n < 0)
    return 0;
  if (n == 0)
    return 1;
  if (m == n)
    return 1;
  return KAI(m) % MOD * modinv(KAI(n) % MOD * KAI(m - n) % MOD) % MOD;
}

ll COM2(ll m, ll n) {
  if (m < n)
    return 0;
  if (n < 0)
    return 0;
  if (n == 0)
    return 1;
  if (m == n)
    return 1;
  return KAI2(m) / KAI2(n) / KAI2(m - n);
}

ll DEC(ll x, ll m, ll n) // xのm進数でのx^nの位の値
{
  return x % POW2(m, n + 1) / POW2(m, n);
}

ll keta(ll x, ll n) // xのn進数での桁数
{
  if (x == 0)
    return 0;
  return keta(x / n, n) + 1;
}

ll DIV(ll x, ll n) // x!のnで割り切れる回数
{
  if (x == 0)
    return 0;
  return x / n + DIV(x / n, n);
}

ll ORD(ll x, ll n) // xのnで割り切れる回数
{
  if (x == 0)
    return INF;
  if (x % n != 0)
    return 0;
  return 1 + ORD(x / n, n);
}

ll SGS(ll x, ll y, ll m) // 1+x+…+x^(y-1)をmで割った余り
{
  if (y == 0)
    return 0;
  if (y % 2 == 0) {
    return (1 + POW3(x, y / 2, m)) * SGS(x, y / 2, m) % m;
  }
  return (1 + x * SGS(x, y - 1, m)) % m;
}

ll SSGS(ll x, ll y, ll m) // Σ[k=1→y](1+x+…+x^(k-1))をmで割った余り
{
  if (y == 0)
    return 0;
  if (y == 1)
    return 1;
  if (y % 2 == 0) {
    return (SSGS(x, y / 2, m) * (POW3(x, y / 2, m) + 1) % m +
            SGS(x, y / 2, m) * y / 2 % m) %
           m;
  }
  return (SSGS(x, y - 1, m) * x % m + y) % m;
}

struct UnionFind {
  vector<int> par;
  vector<int> sizes;
  UnionFind(int n) : par(n), sizes(n, 1) { rep(i, n) par[i] = i; }
  int find(int x) {
    if (x == par[x])
      return x;
    return par[x] = find(par[x]);
  }
  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (sizes[x] < sizes[y])
      swap(x, y);
    par[y] = x;
    sizes[x] += sizes[y];
  }
  bool same(int x, int y) { return find(x) == find(y); }
  int size(int x) { return sizes[find(x)]; }
};

int main() {
  string s, t;
  ll dp[3200][3200], m[3200][3200], a, b;
  stack<char> p;
  cin >> s >> t;
  rep(i, s.length()) {
    rep(j, t.length()) {
      if (s[i] == t[j]) {
        dp[i + 1][j + 1] = dp[i][j] + 1;
        m[i + 1][j + 1] = 0;
      }
      dp[i + 1][j + 1] = max({dp[i + 1][j], dp[i][j + 1], dp[i + 1][j + 1]});
      if (dp[i + 1][j + 1] == dp[i + 1][j]) {
        m[i + 1][j + 1] = 1;
      }
      if (dp[i + 1][j + 1] == dp[i][j + 1]) {
        m[i + 1][j + 1] = 2;
      }
    }
  }
  a = s.length();
  b = t.length();
  while (a >= 1 && b >= 1) {
    if (m[a][b] == 0) {
      p.push(s[a - 1]);
      a--;
      b--;
    } else if (m[a][b] == 1)
      b--;
    else
      a--;
  }
  while (p.size()) {
    cout << p.top();
    p.pop();
  }
}