Problem p03335 (C++)

Buggy submission (status: Runtime Error):
// This amazing code is by Eric Sunli Chen.
#include <bits/stdc++.h>
using namespace std;
template <typename T> bool get_int(T &x) {
  char t = getchar();
  bool neg = false;
  x = 0;
  for (; (t > '9' || t < '0') && t != '-' && t != EOF; t = getchar())
    ;
  if (t == '-')
    neg = true, t = getchar();
  if (t == EOF)
    return false;
  for (; t <= '9' && t >= '0'; t = getchar())
    x = x * 10 + t - '0';
  if (neg)
    x = -x;
  return true;
}
template <typename T> void print_int(T x) {
  if (x < 0)
    putchar('-'), x = -x;
  short a[20] = {}, sz = 0;
  while (x > 0)
    a[sz++] = x % 10, x /= 10;
  if (sz == 0)
    putchar('0');
  for (int i = sz - 1; i >= 0; i--)
    putchar('0' + a[i]);
}
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define get1(a) get_int(a)
#define get2(a, b) (get1(a) && get1(b))
#define get3(a, b, c) (get1(a) && get2(b, c))
#define printendl(a) print_int(a), puts("")
typedef long long LL;
typedef unsigned long long uLL;
typedef pair<int, int> pii;
const int inf = 0x3f3f3f3f;
const LL Linf = 1ll << 61;
const double pi = acos(-1.0);

const int maxn = 2111;

int n, m, seq[maxn], fa[maxn], timer;
vector<int> g[maxn];

void dfs(int x, int f) {
  fa[x] = f;
  seq[++timer] = x;
  for (auto v : g[x])
    if (v != f)
      dfs(v, x);
}

vector<int> get_ans(vector<pii> es, int cur) {
  vector<int> nw;
  nw.resize(es.size(), 0);
  if (cur == 1)
    return nw;
  int piv = seq[cur], val = -1;
  //	printf("piv= %d\n",piv);

  vector<pii> n_es, tp;

  for (int i = 0; i < (int)es.size(); i++) {
    if (es[i].ff != piv && es[i].ss != piv) {
      n_es.pb(es[i]);
      tp.pb(mp(i, -1));
      continue;
    }
    if (es[i].ff == piv) {
      swap(es[i].ff, es[i].ss);
      nw[i] = 1;
    }
    if (es[i].ff == piv)
      continue;
    if (val == -1)
      val = i;
    else {
      n_es.pb(mp(es[piv].ff, es[i].ff));
      tp.pb(mp(val, i));
      val = -1;
    }
  }
  if (val != -1) {
    n_es.pb(mp(es[val].ff, fa[piv]));
    tp.pb(mp(val, -1));
  }
  vector<int> old = get_ans(n_es, cur - 1);
  for (int i = 0; i < (int)old.size(); i++) {
    nw[tp[i].ff] ^= old[i];
    if (tp[i].ss >= 0)
      nw[tp[i].ss] ^= old[i] ^ 1;
  }
  return nw;
}

int main() {
  get2(n, m);
  for (int i = 1, u, v; i < n; i++) {
    get2(u, v);
    g[u].pb(v);
    g[v].pb(u);
  }
  vector<pii> es;
  for (int i = 1, u, v; i <= m; i++) {
    get2(u, v);
    es.pb(mp(u, v));
  }
  dfs(1, 0);
  vector<int> ans = get_ans(es, n);
  set<pii> st;
  for (int i = 0; i < (int)es.size(); i++) {
    if (ans[i])
      swap(es[i].ff, es[i].ss);
    timer = 0;
    dfs(es[i].ff, 0);
    int tmp = es[i].ss;
    while (tmp != es[i].ff) {
      st.insert(mp(fa[tmp], tmp));
      tmp = fa[tmp];
    }
  }
  printf("%d\n", (int)st.size());
  for (int i = 0; i < (int)es.size(); i++)
    printf("%d %d\n", es[i].ff, es[i].ss);
  return 0;
}


Fixed submission:
// This amazing code is by Eric Sunli Chen.
#include <bits/stdc++.h>
using namespace std;
template <typename T> bool get_int(T &x) {
  char t = getchar();
  bool neg = false;
  x = 0;
  for (; (t > '9' || t < '0') && t != '-' && t != EOF; t = getchar())
    ;
  if (t == '-')
    neg = true, t = getchar();
  if (t == EOF)
    return false;
  for (; t <= '9' && t >= '0'; t = getchar())
    x = x * 10 + t - '0';
  if (neg)
    x = -x;
  return true;
}
template <typename T> void print_int(T x) {
  if (x < 0)
    putchar('-'), x = -x;
  short a[20] = {}, sz = 0;
  while (x > 0)
    a[sz++] = x % 10, x /= 10;
  if (sz == 0)
    putchar('0');
  for (int i = sz - 1; i >= 0; i--)
    putchar('0' + a[i]);
}
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define get1(a) get_int(a)
#define get2(a, b) (get1(a) && get1(b))
#define get3(a, b, c) (get1(a) && get2(b, c))
#define printendl(a) print_int(a), puts("")
typedef long long LL;
typedef unsigned long long uLL;
typedef pair<int, int> pii;
const int inf = 0x3f3f3f3f;
const LL Linf = 1ll << 61;
const double pi = acos(-1.0);

const int maxn = 2111;

int n, m, seq[maxn], fa[maxn], timer;
vector<int> g[maxn];

void dfs(int x, int f) {
  fa[x] = f;
  seq[++timer] = x;
  for (auto v : g[x])
    if (v != f)
      dfs(v, x);
}

vector<int> get_ans(vector<pii> es, int cur) {
  vector<int> nw;
  nw.resize(es.size(), 0);
  if (cur == 1)
    return nw;
  int piv = seq[cur], val = -1;
  //	printf("piv= %d\n",piv);

  vector<pii> n_es, tp;

  for (int i = 0; i < (int)es.size(); i++) {
    if (es[i].ff != piv && es[i].ss != piv) {
      n_es.pb(es[i]);
      tp.pb(mp(i, -1));
      continue;
    }
    if (es[i].ff == piv) {
      swap(es[i].ff, es[i].ss);
      nw[i] = 1;
    }
    if (es[i].ff == piv)
      continue;
    if (val == -1)
      val = i;
    else {
      n_es.pb(mp(es[val].ff, es[i].ff));
      tp.pb(mp(val, i));
      val = -1;
    }
  }
  if (val != -1) {
    n_es.pb(mp(es[val].ff, fa[piv]));
    tp.pb(mp(val, -1));
  }
  vector<int> old = get_ans(n_es, cur - 1);
  for (int i = 0; i < (int)old.size(); i++) {
    nw[tp[i].ff] ^= old[i];
    if (tp[i].ss >= 0)
      nw[tp[i].ss] ^= old[i] ^ 1;
  }
  return nw;
}

int main() {
  get2(n, m);
  for (int i = 1, u, v; i < n; i++) {
    get2(u, v);
    g[u].pb(v);
    g[v].pb(u);
  }
  vector<pii> es;
  for (int i = 1, u, v; i <= m; i++) {
    get2(u, v);
    es.pb(mp(u, v));
  }
  dfs(1, 0);
  vector<int> ans = get_ans(es, n);
  set<pii> st;
  for (int i = 0; i < (int)es.size(); i++) {
    if (ans[i])
      swap(es[i].ff, es[i].ss);
    timer = 0;
    dfs(es[i].ff, 0);
    int tmp = es[i].ss;
    while (tmp != es[i].ff) {
      st.insert(mp(fa[tmp], tmp));
      tmp = fa[tmp];
    }
  }
  printf("%d\n", (int)st.size());
  for (int i = 0; i < (int)es.size(); i++)
    printf("%d %d\n", es[i].ff, es[i].ss);
  return 0;
}