最后一个点WA了。

#include<bits/stdc++.h>
using namespace std;
struct line{
	int x,y1,y2;
	int op;
	bool operator<(line s)const{
		if(x==s.x) return op>s.op;
		return x<s.x;
	}
};
struct node{
	int l,r;
	int len;
	int covernumber;
	bool flagl,flagr;
	int ans;
}tree[3000010];
long long ally[1000010];
int cnt,len;
int reply(int x){
	return lower_bound(ally+1,ally+1+len,x)-ally;
}
void pushup(int rt,int l,int r){
	if(tree[rt].covernumber){
		tree[rt].len=ally[tree[rt].r+1]-ally[tree[rt].l];
		tree[rt].flagl=tree[rt].flagr=true;
		tree[rt].ans=1;
	}
	else{
		tree[rt].len=tree[rt<<1].len+tree[rt<<1|1].len;
		tree[rt].flagl=tree[rt<<1].flagl;
		tree[rt].flagr=tree[rt<<1|1].flagr;
		tree[rt].ans=tree[rt<<1].ans+tree[rt<<1|1].ans;
		if(tree[rt<<1].flagr&&tree[rt<<1|1].flagl) tree[rt].ans-=1;
	}
}
void build(int l,int r,int rt){
	tree[rt].l=l;
	tree[rt].r=r;
	tree[rt].len=tree[rt].covernumber=tree[rt].ans=0;
	tree[rt].flagl=tree[rt].flagr=false;
	if(l==r) return;
	int m=(l+r)>>1;
	build(l,m,rt<<1);
	build(m+1,r,rt<<1|1);
}
void update(int l,int r,int opt,int rt){
	if(l<=tree[rt].l&&tree[rt].r<=r){
		tree[rt].covernumber+=opt;
		pushup(rt,l,r);
		return;
	}
	int mid=(tree[rt].l+tree[rt].r)>>1;
	if(l<=mid) update(l,r,opt,rt<<1);
	if(mid<r) update(l,r,opt,rt<<1|1);
	pushup(rt,l,r);
}
signed main(){
	int n;
	scanf("%d",&n);
	int tot=0;
	vector<line> v;
	for(int i=1;i<=n;i++){
		int x1,y1,x2,y2;
		scanf("%d %d %d %d",&x1,&y1,&x2,&y2);
		v.push_back((line){x1,y1,y2,1});
		ally[++tot]=y1;
		v.push_back((line){x2,y1,y2,-1});
		ally[++tot]=y2;
	}
	sort(ally+1,ally+1+tot);
	len=unique(ally,ally+1+tot)-(ally+1);
	sort(v.begin(),v.end());
	build(1,len-1,1);
	long long answer=0;int pre=0;
	for(int i=0;i<2*n-1;i++){
		update(reply(v[i].y1),reply(v[i].y2)-1,v[i].op,1);
		answer+=abs(tree[1].len-pre);
		answer+=(v[i+1].x-v[i].x)*2*tree[1].ans;
		pre=tree[1].len;
	}
	answer+=v[2*n-1].y2-v[2*n-1].y1;
	printf("%lld",answer);
	return 0;
} 

向大佬们求助