90分，最后一个点MLE了，求求dalao们帮忙优化一下qwq

#include<bits/stdc++.h>
using namespace std;
#define int long long
int n;
struct node {
	int xa, ya, xb, yb;
};
vector<int> px, py;
vector<node> add;
const int N = 4010;
int c[N][N], s[N][N];
node idx[N];
signed main() {
	cin >> n;
	for(int i = 1; i <= n; i++) {
		int xa, ya, xb, yb;
		cin >> xa >> yb >> xb >> ya;
		px.push_back(xa);
		py.push_back(ya);
		px.push_back(xb);
		px.push_back(xb - 1);
		py.push_back(yb);
		py.push_back(yb - 1);
		add.push_back({xa, ya, xb, yb});
	}
	sort(px.begin(), px.end());
	sort(py.begin(), py.end());
	px.erase(unique(px.begin(), px.end()), px.end());
	py.erase(unique(py.begin(), py.end()), py.end());
	for(int i = 1; i <= n; i++) {
		idx[i].xa = lower_bound(px.begin(), px.end(), add[i - 1].xa) - px.begin() + 1;
		idx[i].ya = lower_bound(py.begin(), py.end(), add[i - 1].ya) - py.begin() + 1;
		idx[i].xb = lower_bound(px.begin(), px.end(), add[i - 1].xb) - px.begin() + 1;
		idx[i].yb = lower_bound(py.begin(), py.end(), add[i - 1].yb) - py.begin() + 1;
	}
	for(int i = 1; i <= n; i++) {
		c[idx[i].xa][idx[i].ya]++;
		c[idx[i].xa][idx[i].yb]--;
		c[idx[i].xb][idx[i].ya]--;
		c[idx[i].xb][idx[i].yb]++;
	}
	for(int i = 1; i <= px.size(); i++) {
		for(int j = 1; j <= py.size(); j++) {
			s[i][j] = s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1] + c[i][j];
		}
	}
	long long ans = 0;
	for(int i = 1; i <= px.size(); i++) {
		for(int j = 1; j <= py.size(); j++) {
			if(s[i][j] > 0) {
				j++;
				for(; s[i][j] > 0 && j <= py.size(); j++) {
					if(s[i - 1][j - 1] > 0) {
						ans += (py[j - 1] - py[j - 2]) * (px[i - 1] - px[i - 2]);
					}
					else {
						ans += py[j - 1] - py[j - 2];
					}
				}
				if(j <= py.size()) {
					if(s[i - 1][j - 1] > 0) {
						ans += (py[j - 1] - py[j - 2]) * (px[i - 1] - px[i - 2]);
					}
					else {
						ans += py[j - 1] - py[j - 2];
					}
				}
			}
		}
	}
	cout << ans;
	return 0;
}