#include<bits/stdc++.h>
using namespace std;

typedef long long LL;
#define lc(p) (p << 1)
#define rc(p) ((p << 1) | 1) 
const int N = 2e5 + 10;

struct Line {
	int p;
	int st, ed;
	int flg;
} L[N];   // 一条竖线：x 坐标是 p，y 的范围是 st 至 ed 
int s[N * 2], cnt;

struct trnode {
	int l, r, c;
	LL len;
} tr[N << 3];

bool cmp(Line la, Line lb) {
	return la.p < lb.p;
}

void pushup(int p) {
	if (tr[p].c > 0) {
		tr[p].len = s[tr[p].r] - s[tr[p].l];
	}
	else {
		tr[p].len = tr[lc(p)].len + tr[rc(p)].len;
	}
}

void bt(int p, int l, int r) {
	tr[p] = {l, r, 0, 0};
	if (l + 1 == r) {
		return ;
	}
	
	int mid = (l + r) >> 1;
	bt(lc(p), l, mid);
	bt(rc(p), mid, r); // 树里面是区间段不是点，所以不用 r + 1 
}

void change(int p, int l, int r, int c) {
	if (r <= s[tr[p].l] || l >= s[tr[p].r]) {
		// 相等的情况说明只是区间段的边界点擦边
		// 并不代表区间段相交，所以也要 return 
		return ;
	}
	if (l <= s[tr[p].l] && s[tr[p].r] <= r) {
		tr[p].c += c;
		pushup(p);
		return ;
	}
	
	change(lc(p), l, r, c);
	change(rc(p), l, r, c);
	pushup(p);
} 

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	int n;
	cin >> n;
	int xa, ya, xb, yb;
	for (int i = 1; i <= n; i ++) {
		cin >> xa >> ya >> xb >> yb;
		L[i] = {xa, ya, yb, 1};
		L[i + n] = {xb, ya, yb, -1};
		s[i] = ya; s[i + n] = yb;
	}
	
	n *= 2;
	sort (s + 1, s + n + 1);
	cnt = unique(s + 1, s + n + 1) - (s + 1);
	sort (L + 1, L + n + 1, cmp);
	
	bt(1, 1, cnt);
	LL ans = 0;
	for (int i = 1; i <= n; i ++) {
		change(1, L[i].st, L[i].ed, L[i].flg);
		ans += (L[i + 1].p - L[i].p) * tr[1].len;
	}
	
	cout << ans << "\n";
	
	return 0;
}

C15【模板】扫描线算法

#include<bits/stdc++.h>
#define lc(p) (p<<1)
#define rc(p) (p<<1|1)
#define LL long long
using namespace std;
const int N=2e5+10;
struct Line{LL p,st,ed,flg; }L[N];// 一条竖线：x坐标是p，y的范围是st至ed 
bool cmp(Line n1,Line n2){return n1.p<n2.p;} 
LL lsh[N];
struct trnode{int l,r,c;LL len;}tr[N<<3];
void pushup(int p)
{
	if(tr[p].c>0) tr[p].len=lsh[tr[p].r]-lsh[tr[p].l];
	else          tr[p].len=tr[lc(p)].len+tr[rc(p)].len ;
}
void bt(int p,int l,int r)
{
	tr[p]=trnode{l,r,0,0};
	if(l+1==r)return ;
	int m=(l+r)>>1;
	bt(lc(p),l,m);bt(rc(p),m,r);
}
void change(int p,int l,int r,int c)
{
	if(r<=lsh[tr[p].l]||lsh[tr[p].r]<=l)return;
	if(l<=lsh[tr[p].l]&&lsh[tr[p].r]<=r)
	{
		tr[p].c+=c;
		pushup(p);
		return ;
	}
	change(lc(p),l,r,c);change(rc(p),l,r,c);
	pushup(p);
}
int main()
{
	int n;scanf("%d",&n);
	for(int i=1;i<=n;i++)
	{
		LL X1,Y1,X2,Y2;scanf("%lld%lld%lld%lld",&X1,&Y1,&X2,&Y2);
		L[i]  =Line{X1,Y1,Y2, 1};
		L[n+i]=Line{X2,Y1,Y2,-1};
		lsh[i]=Y1;lsh[n+i]=Y2;
	}
	sort(lsh+1,lsh+2*n+1);int ln=unique(lsh+1,lsh+2*n+1)-lsh-1;
	bt(1,1,ln);
	sort(L+1,L+2*n+1,cmp);
	LL ans=0;
	for(int i=1;i<2*n;i++)
	{
		change(1,L[i].st,L[i].ed,L[i].flg);
		ans+=(L[i+1].p-L[i].p)*tr[1].len;
	}
	printf("%lld\n",ans);
	return 0;
}