Problem p03669 (C++)

Buggy submission (status: Runtime Error):
#include <cstdio>
#include <cstring>
using namespace std;
#define S 200001
int deg[200007], vis[200007], Fa[200007];
inline int Find(register int o) { // fprintf(stderr, "%d\n", o);
  return o == Fa[o] ? o : (Fa[o] = Find(Fa[o]));
}
int fastin() {
  register int i;
  register char ch;
  while (ch = getchar(), ch < '0' || ch > '9')
    ;
  i = ch - '0';
  while (ch = getchar(), ch >= '0' && ch <= '9')
    i = i * 10 + ch - '0';
  return i;
}
int main() {
  register int T, n, h, h2, i, a, b, c, d, u, v;
  T = fastin();
  while (T--) {
    n = fastin(), h = fastin();
    memset(deg + 1, 0, sizeof(int) * (h << 1));
    memset(vis + 1, 0, sizeof(int) * (h << 1));
    h2 = h << 1;
    for (i = 1; i <= h2; ++i)
      Fa[i] = i;
    Fa[S] = S;
    for (i = 1; i <= n; ++i) {
      a = fastin(), b = fastin(), c = fastin(), d = fastin();
      u = (d ? d + h : b);
      v = (c ? c : a + h);
      ++deg[u], vis[u] = 1;
      --deg[v], vis[v] = 1;
      Fa[Find(u)] = Find(v);
    }
    // for (i = 1; i <= h2; ++i) fprintf(stderr, "%d %d\n", deg[i], vis[i]);
    // fprintf(stderr, "GG");
    for (i = 1; i <= h; ++i) {
      if (vis[i]) {
        if (deg[i] < 0)
          break;
        if (deg[i] > 0)
          Fa[Find(i)] = S;
      }
    }
    if (i <= h) {
      puts("NO");
      continue;
    }
    // fprintf(stderr, "HH");
    for (i = h + 1; i <= h2; ++i) {
      if (vis[i]) {
        if (deg[i] > 0)
          break;
        if (deg[i] < 0)
          Fa[Find(i)] = S;
      }
    }
    if (i <= h2) {
      puts("NO");
      continue;
    }
    // fprintf(stderr, "II");
    for (i = 1; i <= h2; ++i)
      if (vis[i] && Find(i) != S)
        break;
    // fprintf(stderr, "%d\n", i);
    if (i <= h2)
      puts("NO");
    else
      puts("YES");
  }
  return 0;
}

Fixed submission:
#include <cstdio>
#include <cstring>
using namespace std;
#define S 200001
int deg[200007], vis[200007], Fa[200007];
inline int Find(register int o) { // fprintf(stderr, "%d\n", o);
  return o == Fa[o] ? o : (Fa[o] = Find(Fa[o]));
}
int fastin() {
  register int i;
  register char ch;
  while (ch = getchar(), ch < '0' || ch > '9')
    ;
  i = ch - '0';
  while (ch = getchar(), ch >= '0' && ch <= '9')
    i = i * 10 + ch - '0';
  return i;
}
int main() {
  register int T, n, h, h2, i, a, b, c, d, u, v;
  T = 1;
  while (T--) {
    n = fastin(), h = fastin();
    memset(deg + 1, 0, sizeof(int) * (h << 1));
    memset(vis + 1, 0, sizeof(int) * (h << 1));
    h2 = h << 1;
    for (i = 1; i <= h2; ++i)
      Fa[i] = i;
    Fa[S] = S;
    for (i = 1; i <= n; ++i) {
      a = fastin(), b = fastin(), c = fastin(), d = fastin();
      u = (d ? d + h : b);
      v = (c ? c : a + h);
      ++deg[u], vis[u] = 1;
      --deg[v], vis[v] = 1;
      Fa[Find(u)] = Find(v);
    }
    // for (i = 1; i <= h2; ++i) fprintf(stderr, "%d %d\n", deg[i], vis[i]);
    // fprintf(stderr, "GG");
    for (i = 1; i <= h; ++i) {
      if (vis[i]) {
        if (deg[i] < 0)
          break;
        if (deg[i] > 0)
          Fa[Find(i)] = S;
      }
    }
    if (i <= h) {
      puts("NO");
      continue;
    }
    // fprintf(stderr, "HH");
    for (i = h + 1; i <= h2; ++i) {
      if (vis[i]) {
        if (deg[i] > 0)
          break;
        if (deg[i] < 0)
          Fa[Find(i)] = S;
      }
    }
    if (i <= h2) {
      puts("NO");
      continue;
    }
    // fprintf(stderr, "II");
    for (i = 1; i <= h2; ++i)
      if (vis[i] && Find(i) != S)
        break;
    // fprintf(stderr, "%d\n", i);
    if (i <= h2)
      puts("NO");
    else
      puts("YES");
  }
  return 0;
}