Language: C++

#include <bits/stdc++.h>

using namespace std;

#define all(c) ((c).begin()), ((c).end())
#define dump(c) cerr << "> " << #c << " = " << (c) << endl;
#define iter(c) __typeof((c).begin())
#define tr(i, c) for (iter(c) i = (c).begin(); i != (c).end(); i++)
#define REP(i, a, b) for (int i = a; i < (int)(b); i++)
#define rep(i, n) REP(i, 0, n)
#define mp make_pair
#define fst first
#define snd second
#define pb push_back

typedef unsigned int uint;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<string> vs;
typedef pair<int, int> pii;

const int INF = 1 << 29;
const double EPS = 1e-10;

double zero(double d) {
    return d < EPS ? 0.0 : d;
}

template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
    return os << '(' << p.first << ',' << p.second << ')';
}

template<typename T>
ostream &operator<<(ostream &os, const vector<T> &a) {
    os << '[';
    rep(i, a.size()) os << (i ? " " : "") << a[i];
    return os << ']';
}

// to avoid error on mingw
string toString(int i) {
    stringstream ss;
    ss << i;
    return ss.str();
}

const int MOD = 1000000007;
// a^k
ll fpow(ll a, ll k, int M) {
    ll res = 1ll;
    ll x = a;
    while (k != 0) {
        if ((k & 1) == 1)
            res = (res * x) % M;
        x = (x * x) % M;
        k >>= 1;
    }
    return res;
}

int compress(vector<pii>& p, int w) {
    vi xs;
    rep(i, p.size()) {
        REP(d, -1, 1+1) {  // -1, 0, 1  // 蟾ヲ蜿ウ繧貞性繧√※菫晏ュ?            int tx1 = p[i].first + d, tx2 = p[i].second + d;
            if(0 <= tx1 && tx1 <= w) xs.push_back(tx1);
            if(0 <= tx2 && tx2 <= w) xs.push_back(tx2);
        }
    }

    // 驥崎、?ｒ蜑企勁
    sort(all(xs));
    xs.erase(unique(all(xs)), xs.end());

    // 蝨ァ邵ョ縺励◆蛟、縺ォ譖ク縺肴鋤縺?    rep(i, p.size()) {
        p[i].first = find(xs.begin(), xs.end(), p[i].first) - xs.begin();
        p[i].second = find(xs.begin(), xs.end(), p[i].second) - xs.begin();
    }

    return xs.size();
}
int dx[] = {1, 0, -1, 0};
int dy[] = {0, -1, 0, 1};
int main() {
    cout.setf(ios::fixed, ios::floatfield);
    cout.precision(8);
    ios_base::sync_with_stdio(false);

    int w, h, n;
    cin >> w >> h >> n;

    vector<pii> px(n), py(n);
    rep(i, n)
        cin >> px[i].first >> py[i].first >> px[i].second >> py[i].second;

    w = compress(px, w);
    h = compress(py, h);

    w--, h--;

    // 蝪励ｊ縺、縺カ縺?    vector< vector<bool> > fld(h, vector<bool>(w, false));
    rep(i, n) {
        REP(y, py[i].first, py[i].second) {
            REP(x, px[i].first, px[i].second)
                fld[y][x] = true;
        }
    }

    int res = 0;
    rep(r, h) {
        rep(c, w) {
            if(!fld[r][c]) {
                res++;

                // bfs
                queue<pii> que;
                que.push(mp(r, c));
                while(!que.empty()) {
                    pii now = que.front(); que.pop();
                    fld[now.first][now.second] = true;

                    rep(d, 4) {
                        int my = now.first + dy[d];
                        int mx = now.second + dx[d];
                        if(0 <= my && my < h && 0 <= mx && mx < w &&
                           !fld[my][mx])
                            que.push(mp(my, mx));
                    }
                }
            }
        }
    }

    cout << res << endl;

    return 0;
}

Compiler output:
a.cc: In function 'int compress(std::vector<std::pair<int, int> >&, int)':
a.cc:70:21: error: 'tx1' was not declared in this scope
   70 |             if(0 <= tx1 && tx1 <= w) xs.push_back(tx1);
      |                     ^~~
a.cc:71:21: error: 'tx2' was not declared in this scope
   71 |             if(0 <= tx2 && tx2 <= w) xs.push_back(tx2);
      |                     ^~~
a.cc:80:11: error: 'i' was not declared in this scope
   80 |         p[i].first = find(xs.begin(), xs.end(), p[i].first) - xs.begin();
      |           ^
a.cc:82:5: warning: no return statement in function returning non-void [-Wreturn-type]
   82 |     }
      |     ^
a.cc: At global scope:
a.cc:84:5: error: expected unqualified-id before 'return'
   84 |     return xs.size();
      |     ^~~~~~
a.cc:85:1: error: expected declaration before '}' token
   85 | }
      | ^
a.cc: In function 'int main()':
a.cc:109:17: error: 'fld' was not declared in this scope
  109 |                 fld[y][x] = true;
      |                 ^~~
a.cc:116:17: error: 'fld' was not declared in this scope
  116 |             if(!fld[r][c]) {
      |                 ^~~